Sometimes one family of products won't work well with another. For example, when I have Coll 845 on my car, the BFRW doesn't work very well and is tough to get streak free, but Duragloss RW works fine. The only way to know if it will work the way you want it to, is if you try it out (or maybe someone here uses Ultima RW with BF on the paint and can tell you).
Agree with Robert on the waterspots...it depends what type. I did a car this weekend with Type II waterspots. I actually didn't even see them until after I polished because of all the swirls and holograms that were on top of the spots. But I used PF2500 and a white pad on GG6 to polish the car and that didn't remove them. So if what you have is Type I, you might be able to get them off with GEP.