They should be ok; a yellow is as good a tang as any to put with a PB. It is also a plus that you're putting the PB in last, they can be extremely territorial and aggressive to newcomers. As was mentioned, the yellow will probably have the whole tank as his territory, but they should settle in. I don't think the size will matter much, as long as the pb isn't a "tiny" one Like some of the quarter-size regals on the market. BTW; I don't like to relate my experience as a rule; because someone has always kept every possible combination of fish together. But, I've kept these two fish together at least 3 times with no problems. I have 3 yellows and a PB in my big tank now.