jaimoe said:
Hey Becca if you don't mind me asking, how many fish do you have in your 75 gallon? I've only got ten cichlids in my 75 and I'm starting to wonder if some of the aggression I've seen lately is due to undercrowding.....
I don't mind you asking, jaimoe - just remember that I'm learning too, so nothing I say or do should be taken to be the absolute end-all, be-all of fish advice.

If someone says something that contradicts what I have, by all means, listen to them as well. There's a lot of great advice on this forum, but even still, I find the best way to setup a great aquarium and learn more is to find out what works for *you*.
That having been said, I have 21 African Cichlids, 2 Catfish and 1 nicely sized pleco.
My Cichlids range in size and agression. I've got everything from the more agressive line, such as Demasoni, Kenyi and Johanni to the less agressive fish, including Yellow and White Labs. Some of my mix is not ideal - if you're looking at breeding purposes - but my tank is purely show and I have no interest (yet) in breeding any of the fish I own. If they have babies, and the babies survive - great. If not, then no loss.
As for the catfish - I've tried a couple of fits - but the ones that have outlasted everyone are my beloved Synodontis Batensodas - a variation of Upside Down Catfish. I love these guys.

(I did have Clown Loaches, but lost them to due to an unfortunately Ammonia spike while I was on vacation - word to the wise, avoid the sinking vacation pellets.)
And the pleco - he's just magnificent. I had a King Tiger pleco in my tank for quite a while, but when I got the bigger common, I switched him out to my baby South American tank.
Hope this helps answer your question!
Edit: Also, I have a lot of decorations and hiding places - but I'm careful to not keep enough areas for everyone to claim their own territory. I read somewhere that a key to help controlling agression in a tank is 1) Overstock and 2) Either provide enough places for everyone to have their own territory or keep the amount of caves, holes, etc to a minimum so the fish don't have a chance to claim their own area.
Personally, in my tank, I have 2 Lava Rocks with TONS of holes in them - the fish love swimming through these. I have 1 nicely sized cave, a couple pieces of slate laying loose buried in sand, two pieces of "holy rock" from PetSmart and a few plastic plants hanging around.