it seems that it has to do with the school's makeup. If you have a school of clowns and only one or two Y.mods the Y. mods will calm down and sort of join in, but if you have a school of Y.mods and one or two clowns the clowns will go billy-bad-ass to try to keep up. Either way, they won't make one happy school. They have very different personalities and seem to speak different languages.
Try getting 3 or 4 more of each and see if they'll live their independent lives (needs BIG tank >100 or 150Gal), or give one away and concentrate on the other species... which also need >100Gal, sorry. Both species are big, active animals.
A