When I was newer to the hobby a LFS employee told me that mixing gold/blue/opaline/pearl gouramis would work just fine in a 90 gallon tank. They said that gouramis were very peaceful community fish. They also recommended some other fish to go with them. The result was that one of the gold gouramis and one of the blue paired off and started terrorizing (and killing) everything in the tank. The problem was that whenever I was in the room, they all hung out looking completely peaceful and I could not figure out what was causing all the deaths in my tank. Also, I usually found the fish bodies after other tank mates had been snacking, so I wasn't sure if they had been mangled before or after they died. I also didn't know as much about what to look for at the time. I finally caught them at it by hiding in the corner of the room with the room's lights off waiting to see if anything happened. I discovered all the gold/opaline/blue gouramis were pretty aggressive, and that the breeding pair were downright brutal. I returned all of the fish to the LFS the next day, but not before they had killed 2 pearl gouramis, 2 angelfish, and some guppies.
I do however love pearl gouramis. They seemed much more peaceful than the others.
Lucky for you that you know where to ask opinions before you get the fish, rather than just trust one person at an LFS.