Yes, there are certain language features that exist in C# that are not in Unityscript, but you should have no trouble. However I wouldn’t recommend mixing scripts like you say, I would just create a “Movement” script in which ever language you prefer. If later you decided to use a 3rd party script written in another language you should have little issues adding that script to your game. Just make sure you don’t have a Unityscript and a C# script with the same name as that will cause issues when building.
Yes. It should be. This is object oriented so both scripts should be running at once if you put it on the same object. Or something like that.