At that tank size, almost all triggers could be kept together, although it's a risk to try the most notoriously aggressive ones such as clowns and queens. There are quite a few people who keep queen and clown triggers with other triggers and together in 180g tanks and get away with it, so in a 400g I dont see why you couldnt. If you dont want to have to worry about a potential war or rehoming going on in your future, I'd just skip the triggers that are known to be tankmate killers.
So basically, you could mix and match any triggers you like in there, imo. It would all depend on the rock work and the size of the triggers though. If you bought full show size adult triggers, I'd think you would be a lot less successful in your endeavor. If you buy a few triggers all in the 3-4" range, you've probably got a long time until you even begin to have to worry about aggression. I kept 5 triggers in my 210 successfully and saw almost no aggression at all. Once or twice a week I'd see my clown chase away humu or something of that nature, but there was no biting or attacking, just some tail posturing and the other guy would head the other direction.