I would think he would like a piranha tank best, but a 29 gallon tank is too small. You really need to breed your own feeders rather than buy the disease ridden goldfish from the store. Only a matter of time before you introduce parasites into your tank, some quite pesky like anchorworm, others deadly dangerous like Camallanus worms and gill flukes which are hard to get rid of.
Personally, I suggest making the 29 gallon tank a livebearer tank so you can breed your own feeders (mosquitofish are great, no heater required, very low maintenance) and getting a bigger tank for predatory fish. Since you were thinking of gars, maybe halfbeaks can be special ordered from your pet store?
You should be aware, also, that a diet of mostly other fish and things like shrimp, is very high in Thiaminase, an enzyme which destroys Vitamin B1 (Thiamine). Gut loading the fish helps, but only so much. Try supplementing with snails, small crayfish, earthworms, crickets, etc. Variety is the key.
If you're interested, here's a whole article that explains the Thiaminase issue:
http://www.wetwebmedia.com/ca/volume_6/volume_6_1/thiaminase.htm