So with my 18-70 it takes a 67mm filter, the 50mm I wanna get takes a 52mm, is it ok to just get a step up ring? and use my current filter?

You could (I own several step up/down rings), but I'd just buy a dedicated filter instead. Only reason I own step rings is to stack lens for macro photog without buying an actual macro lens.

So with my 18-70 it takes a 67mm filter, the 50mm I wanna get takes a 52mm, is it ok to just get a step up ring? and use my current filter?

You will probably be using the 50mm a lot. Switching filters and and rings will get old. Also if you plan on leaving the filter on for extended shooting you might like to have a lens cap the size of the filter.

I didn't even know they made step-up or step-down rings for filters. I'd just assume buy the right size filter for the lens.

Do DSLR lenses require clear filters? I thought I read somewhere that they don't, but the oldschool 35mm film cameras do.

I put filters on just as much for the optic protection than the effect the filter provides.