i actually have a 55 gallon tank, which i just put 2 convicts and a salvini in...there are alot of different cichlids you could keep in here
firemouths, jack dempsey, Cons, Salvinis, Severums (i think)
as for the tiger barbs, they could go with them as dithers yes, but with the aggressiveness the convicts have there is no guarantee they will make it, it all depends on the personality of the convict or other agressive fish
If you have a pair of convicts, then there's little point in mixing in other fish. Everything I can think of will be either too passive for breeding convicts or too aggressive for convicts without fry.
I suggest trying one convict and a large school of tiger barbs. Maybe a couple of bristlenose plecos. Be sure to include lots of ground cover: rocks, terracotta pots, driftwood, etc. Otherwise you won't get as much territory defense.
Although, now that I think about it, why does your friend want an aggressive tank? If he just wants to see fish behaving territorially, then he might want to get Mbuna cichlids (and a bunch of rocks). If he wants to have fish that try to attack him, then get a breeding pair of convicts, dempseys, or jewel cichlids. If he wants a predatory fish, well, most of those aren't actually that aggressive; they tend to be ambush predators and just lurk around.
In any case, you'll get a livelier tank if you give the fish things to hide around and defend. It sounds counter-intuitive, but providing ample hiding places tends to make fish feel more secure and more comfortable about being active in the open. Similarly, territorial fish like having landmarks that they can defend against encroachment.
But that's enough blathering. If you can tell us why your friend thinks that he wants an "aggressive" tank, then we can help you figure out what sort of fish to get.
In my 55 aggressive i had a school of 7 exodons, a firemouth, a small flowerhorn, a turquise severum, and a pictus catfish. I enjoyed all these fish and would suggest any of them for an aggressive tank.
then exodons are great if he wants something lively; piscavorous puffer if he wants a solitary lurker. convicts and other cichlids will just slowly beat things to death.
you might want to get this moved to Predatory Fish before you start getting flames (although most of those people frown on cruelty to feeders)
also, there are a fair number of larger shrimp that can be quite vicious. someone was posting a while back about some red claw shrimp that grew huge pincers and killed everything that didn't kill them first. maybe your friend would be interested in something like that.