Actually I should have read the fish list a bit better.
You'll still be able to add the chromis but only about 4, 6 would have been ideal for schooling. I didn't notice the two species of clowns in the original post. The tangs needs will consume much of the 75's extra space but the main issue will be with the clowns. Four different species of fish being from the same family, you will get some squabbling. Most likely from the Tomato but I would watch things carefully after the new fish pass QT
. Chromis are pretty passive but they will get aggressive if needs be.