I don't own the UM56 yet, but if they'll fit Westones, they'll fit a lot of IEMs. They won't fit the UE Triple Fi Pro, without pulling out the little tube connector, because UE uses sound tubes with a wider diameter. They may not fit the PFE, whose sound tube also has a slightly different length and width. I found it possible to slide on silicon/rubber tips from other makes but not different foamies. The foamies, which need more structure in the center, come with plastic tubes that don't play well with the PFE sound tube. You can't squeeze on into the other.
I don't know how bendable the custom tips are, whether they're closer to silicon or plastic. If it's the former, you should be able to slide them onto practically anything. If it's more like the latter, they should fit most IEM, except for IEMS like the UE and the PFE. Correct me if I'm wrong but Westone, Shure and Etymotic all use the same size sound tube. That's one reason I've decided to buy the UM56 tip. It's an awesome idea and an awesome value. Why customize the whole IEM (at considerable cost) when you can customize the tip for much less?