I made buttercream icing last week and have stored in the fridge. I'm thinking of making it cookies and cream. Would it be ok to add the crushed cookies to what I have already made and just incorporate?

Yes, it will work just fine.

Yes I've done this quite a bit and it's really good, my most requested for kid's cakes, actually.

I mkae this all the time, it is one of my most requested fillings. I just mash up some Oreos and stir them into the buttercream.