Onyx clowns in rose bubble?

I would say that there is a good possibility that the RBTA will host the clowns.

You're right, there is no guarantee that the RBTA will host the clowns.

Bubble tips are one of the more common anemones to host clowns. Onyx clowns are A. ocellaris, which usually will take up residence in a hosting bubble tip.

That being said, I've yet to have an Ocellaris ever turn down a hosting bubble tip.

There are some tricks to get them into the anemone if they aren't willing to go in on their own...

The Onyx Ocellaris aren't that rare, not as common as the basic orange and whites, but the color morphs are becoming more and more popular. It depends on whether you're looking for completely back and white, or still want some orange coloration, etc. These are ORA's, this site sells them: http://www.saltwaterfish.com/ORA-Black-amp-White-Clownfish--Aquacultured_p_226.html

Once an onyx is an onyx, they don't change back to orange.

It has nothing to do with the anemone that they're in. It's the quality of breeding stock, water parameters and food.

For example, right now I have baby ORA Black Ice Clowns. There were some people complaining that they're not changing completely black and white, others were getting it to happen within a few weeks of owning them. Mine changed immediately, but a friend of mine who had a pair from the same batch couldn't get his to fully change. I started doing some research into it and found that people who were seeing a change/quicker change had a higher salinity, and were feeding better foods. I was feeding my clowns Emerald Entree, PE Mysis both soaked in VitaChem and live Mysis and my salinity was 1.025 constant.  They're also in a hosting BTA. My friend only fed flake and occasionally Brine shrimp and kept the salinity at 1.021. The more I looked into it, the more I found it to be true. 

Just be careful with carpets, they like to walk around

Same as most any other anemone, high light, good water quality, stable parameters. They're fine, I just usually don't suggest them because they're sting tends to be more potent than some other anemones, and given that they move around more, they can really do some damage to corals. If putting in a smaller tank, this, combined with their larger size, can really be an issue.