I think a diverse empire would make an excellent Tall empire. Consider - you could colonize almost any planet with 80%ish happiness. There are seven types of habitable worlds, so if your initial Tall empire grabs five worlds of your homeworld type, then as you fill out your diversity you can get ~30 total worlds in that five-world border, assuming average distributions. Along with edicts and policies that boost happiness/productivity/etc for diversity (charisma is a trait, but I wouldn't be surprised if there were other things) you could offset divergence through tolerance happiness, and you'd have a stronger, more defensible core than a sprawling empire that, because it only has one or two species, can only colonize a third of habitable planets.