i have no experience keeping cichlids, but ive heard shelldwellers are good for tanks this size. they are very small and like the name says, they live in shells. i have also heard they are very entertaining. i hope this information is helpful to you.
none are really aggressive but they all are aggressive in their own ways. you are really limited by the tank size to what you could feasibly keep.you can't have a bunch of fish that will get 6-10" as adults in there.
lamprologus ocellatus Kribensis (yes, it's a cichlid from africa!)
3-4" range Demasoni
I can't think of any more but everyone else gets 6" and bigger and not a good fit for 34 gallons unless you just want to keep one or two fish.
These smaller guys you can keep a few- handful or more of in the 34g, or a couple on the 3-4" range instead. anything else will get crowded and bad when they grow up. I'm sure there's a few more in the 3-4" range I can't think of that I missed.
you could probably keep some yellow labidochromis in there, but i have no experience with them some i don´t really know and am guessing from internet research and a friend who told me ¨you can put 3 labidochromis in a standard 20 gallon¨.
Most of your traditional "African Cichlids" (mbuna, Peacocks and Haps) are going to be way too big and require a much larger tank footprint (4ft minimum) that what you will have with 34 gallons.
Putting aggressive African cichlids in a 34 gallon tank is going to create a disaster. You will end up with a bully and the rest will get bullied.....to death. Yes, they will stress out, stop eating, hide....and then die....if the bully doesn't attack and kill them first.
If you want cool Africans in 34 gallons i would stick with shell dwellers only.
What about a pair of Julidochromis ornatus? But really whether you go for Africans (rift lake) or South Americans would depend mainly on you water parameters. Hard water with high pH - rift lake or Central American. Soft water neutral or low pH South American or kribensis or African butterfly cichlid..