I have Merfolk with tails that are just 2 legs they hold together in the water. What I want to know is that by giving them legs(and making them able to walk on land) did I make them more susceptible to other species of water based animals? For the following reasons:
Also, If they are more susceptible is there any way to combat this? Without having them leave the water?
My reasons being that I want them to survive in the water for at least 2 centuries(because of my plot) and I don't want them to have any evolutionary changes.
My Merfolk are very smart with IQ's of at least 200(Genius level) which means that maybe they could invent something. But the "what" they could invent is beyond me?
There are really two questions here: 'will this make them worse at swimming?' and 'will this be enough of a problem to kill them all off?'
The answer to the second one is easy: no, it won't.
Humans aren't exactly helpless in the water, after all, and your merfolk are both quite a lot smarter and (presumably) better swimmers than we are. Working together would make a huge difference, as would a few simple inventions: fin, spear, fishing sling.
If they're smart enough and technologically advanced enough there's no end to the fancy tricks that are available, but they don't need anything like that at all - a long pointy stick does a lot to level the playing field all on its own.
The first question is where things get interesting.
Having a tail made up of two parts instead of one isn't necessarily going to slow them down at all, but there are a few places where there are potential costs to the arrangement (and at least one potential benefit).
If they don't have flukes or something similar, they'll lose quite a lot of speed and control. This is the major issue. Whether it's their feet, flaps of skin on the ankles, or something else, they need to be able to duplicate the shape of a whale or dolphin's tail. (Or a seal's hind legs.) That broad, flat area plays a very important role in both propelling them forward and in letting them manoeuvre - it's effectively a combination propeller and rudder. If they don't have it, they're at a major disadvantage.
If water can pass between their legs as they're swimming, they lose power. If there's a gap where water can pass between their legs, they've effectively got two smaller 'tails' instead of one big one - and two half-sized tails don't add up to the same power as one full-sized one would have. If they can stop the water passing between their legs by pressing them together they get the power back, but it will mean they have to use extra energy while swimming and might tire more quickly. A monofin helps, as does wrapping the legs together.
If they have long leg bones instead of a spinal cord in the 'tail', they lose some effectiveness. A whale's tail can flex at any point along the length, and can bend in any direction. If you replace that with a single knee joint that only bends in one direction it costs you a tiny bit of efficiency for swimming forward, and a bit more for manoeuvring. (On the other hand, a rigid leg bone might be easier to brace to make sharp turns at top speed)
Being able to move their legs independently could make them more manoeuvrable. Being able to separate their legs gives them two independent control surfaces, instead of just one. While a single larger surface is more effective for simple turns (as noted above), it's not implausible that there are some complex manoeuvres that two legs can perform but a single tail couldn't.
I am fairly certain that highly intelligent people could handle predators by grouping together and killing the predators. This has been generally successful on land and should work in water as well. There might be occasional failures, but overall population grew before we began actually exterminating predators.
That said, there's no reason why merfolk couldn't invent open submarines that travel through water. That would allow them to travel much faster than normal. These would be easier than real submarines, as they wouldn't need to be air/water tight. Your merfolk can extract oxygen from water, right?
If you need them to start out primitive, you could start with something pedal driven. Properly geared that should give good bursts of speed and the structure could help protect against large predators.
That should be enough to handle Earth predators. If your world is more difficult, then you'll need to tell us more about what makes it challenging.
Your concern regarding evolution is irrelevant. Assuming your merfolk have a lifespan on the same order of magnitude as humans, or any known animal, two centuries is negligible in relation to an evolutionary timescale. It would seem strange for them to have adapted any aquatic favoring appendages without significant time in the water previous to this two century span. What is their history in this respect?
You already gave the answer in your question - intelligence.
With intelligence comes the power to change themselves and their environment. Their supreme adaptability (access to both sea and land) is also a very strong benefit, expect them to live in coastal areas and access resources of both terrain types. For example harvesting wood from the land for spears that are used to hunt fish in the sea.
For swimming they would need at the minimum flippers, whether these are natural and can fold up to move on land or are artificial is really an artistic decision either would work perfectly well.
They may well develop some sort of "skirt" made of flexible elastic material that they wrap around their legs for underwater work. This will turn their legs into a single tail without needing muscle strength to do so. Alternatively they may evolve webbing between their upper legs combined with feet that can grasp onto each other. When they enter water the feet grab each other to lock together, the same action releasing flukes that spread out and allow them to swim far better than a human.
It is not too reasonable to try to predict what a large group of extreme geniuses would come up with when their lives are at stake. While it's nice to begin the train of thought, please know that even the result of the hardest work by everyone on Stackexchange would at best be cute in comparison.
The IQ is, and has now been for a while, defined by mapping test scores to a normal distribution with mean 100 and standard deviation 15.
Over 6.5 Sigma! Over 6.5 Sigma! I don't like the way IQ is measured, but in this case it really doesn't matter. The average of these beings is probably smarter than any of the 7'000'000'000 humans alive!
Seriously, I have no idea what this folk would do, but I'm pretty convinced that the US military would be terrified not to upset them.
This will come down entirely to the beginning of their civilization and their lack of hands. They'll be weak for a brief period: until they figured out how to manipulate their surroundings and made tools for it. Then, they'll ramp up to extreme levels of organization and technology, without any of the pointless set-backs human society had.
These creatures will quickly develop mathematics and the scientific principle. You don't reach "genius level" without these.
They will quickly agree on laws that heed game theory. Their laws will be written and used by people who are at least on the level of a good economist today; the population will understand their merits and consequences, and cooperate to enforce them.
They will know the value of information and teaching it. Any set-backs will be but experience for future endeavors.
We've just broken through the limiting factors of human society, before even inventing any serious technology.
Nobody in this thread is qualified to say what these people can do. Widespread intelligence is an extreme game changer, and it's hard to predict what it can do by its very nature. Virtually anything crumbles before higher intelligence, anyone who denies that hasn't been looking.