[@pyroman]

When you use your powers you can alter things around you, such as if you want to grab something first or hide something that can happen because everyone will be moving slow. You will be fast in comparison but you would need to return to the same spot when the power stops or you might have more explaining to do than you want.
 There are limits to how long use can use the power though.
Yes they can cause physical and mental strain if you push yourself too far. If you try to hold back time too long there will be a punishment on your body. If you try to go too far back in time, a punishment will happen. If you hold back time when it is trying to start up again ect.
You are most effected by your own powers. As for the others powers it is a no. The other peoples powers will not effect you kind of. I mean it will change things or repeat things around you but it won't effect you as it happens just the after effect.
A minimum of two would be nice more would be better, one main and one side.