They won't have trouble with each other until puberty in a few years. Then 2 males will fight. A female will be sexually harassed and will have to be separated except while mating.
As for space: there are a bunch of popular misconceptions about space. Those misconceptions are based upon the setups used by breeders to keep their stock. Those space notions will allow your animal to live for a few months or a couple of years, though not with pleasure. It's sort of like a criminal living in a cell. You can survive it, but are you happy?
If you think abut it, a turtle out in the wild has an entire pond (whatever the size) to live in and wander around---much more space than the average person can devote to an aquarium.
So give your turtle as much space, plants, structure, and interesting places im which to play as you can afford.
You can never give the animal too much space. (You may give it so much that it will hide and you won't see it anymore---but your animal won't mind; YOU will. What fun is an animal that you can't see!)
So find a happy medium.