im not sure if there are any geos that stay small enough to have a proper school in a 55g, def not altifrons they get too big, i think your best bet would be maybe red heads or something, i think they only get 8in... gymnos stay smaller and are still great looking fish and you could even keep them without a heater, since its going to be at a church might be nice to have it as low maintenance as possible... personally i would do like a big male severum and a bunch of tetras with a few pictus or some other cat on the bottom