Ok, first of all there is no particular species of fish that will be 100% safe in this situation. The problem you have is not only a 6" clown trigger, but an ESTABLISHED 6" clown trigger. That is an important distinction to make.
Another trigger might be a bad choice, but you can never say for sure without trying it. Groupers, morays, puffers, larger angels, larger tangs and lion fish are all possibilites.
The problem is that the clown trigger considers the entire tank his territory, and clown triggers should generally be added last and when they are smaller than everything else. Even then, they usually require thier own tank eventually.
The short version is that you might luck out, but the odds are not good.
Jim