its not advised to do that, models downloaded will be.
different styles
different polycounts
different textured methods.
different texture file formats and sizes.
different limbed sequences.
usually not animated.
mostly different geographical sizes and thats detrimental with classics collision system because if you scale it the original size is the collision space.
I dont think pro suffers from that.
The best thing to do is to learn how to make them for yourself and choose a style that you feel comfortable with. IF your trying to go for super realism straight away, forget it. that takes a tonne of skill so choose something you can create and work with at your current artistic level.
I reccomend milkshape / unwrap3d and maybe characterfx if its not been abandoned by the author.
TGC let us have some darkmatter models over christmas time which might be a good place to start, animated viking etc..

fall down seven times, stand up eight