Uaru are great tankmates for altifrons, I wish I could find some here! I'd skip the silver dollars though, IMO you'll lose the nice long fins from the geos. Most tall bodied tetras should be ok, I keep my large altifrons with lemon tetras and haven't noticed any going missing. Clowns should work too, but get fairly big and can be quite competitive for food with the likes of the geos and royal plec you mentioned that prefer to graze. With the geos being the main feature fish you could look at most species of small-medium sized pleco (or some of the larger ones, depending on how much space you want to dedicate to them), uaru, severum, angels, festivum, corydoras, most tetras, or possibly another species of eatheater like Satanoperca (but IMO just stick with the altifrons). Plenty more options if you look outside South America too...