The only thing I don't like off of your profile list is the two damsels (personal opinion), but two tangs should be fine in the 125. But yes, if they are similar in size, the YT has an advantage, and has an attitude by nature. You can try to slowly introduce (using a cage/separation partition/etc) and if it doesn't go well, then remove the YT. You mentioned that you might get rid of the YT anyway, but I would give them a shot at each other first. While introducing, keep them apart, but make sure they can see each other visually. The yellow will probalby go ballistic for the first 1/2 hour, but hopefully will calm down.