I'd go with something smaller than a florida gar unless you are going to get a huge tank or pond. (on the order of at least 4-5' wide and 8-15' long).
There are like a bazillion kinds of catfish you could use, lima shovlenose size or smaller.
Any of the smaller bichir would be great (polypterus delhezi/sengegalus/retropinnis/mokelembembe/palmas).
If you wanna try some impossibly hard to find fish you could try a hoplias. I think hoplias brasiliensis is small enough. Maybe microlepis and microcephalus. Dunno how big patana or teres get.
Or if you want some truly impossible to find fish you could try and find a hydrocynus tanziniae (smallest african tiger fish). If you find one let me know. I'm beginning to think they are a myth.