How territorial is your yellow tang? Both the Saddleback and Pearscale are both good choices as far as Butterflies (which almost means nothing is a guarantee). Both adapt to tank live pretty well. The Saddleback is out of Hawaii, but has two catches, most of them are collected too big, and as with most large adult fish, don't adapt like the young'ens. The other catch is they grow quite large for a butterfly, up to 9". The max on the Pearlscale 5.5".
If you had more rock (and we knew your filtration), I'd say you honestly could do both butterflies, if you found a small Saddleback. Butterflies are not aggressive towards each other in the sense like Angels. But my recommendation for now would be the Pearlscale, and when the 125 gets here add some more rock and go from there.