You can go the Planet Catfish's Cat-elog and filter it by size and then see which ones you do like and then see which ones are available to you.
16"+ hardly would be fine in a 125, even 180 may not be big enough, especially for an active swimmer (day or night).
I'd stay at 1' and under. A school of dozen sun cats looks awesome IMHO when they are larger. Gulpers are cool but often pose problems for keepers. They don't move much except when it is feeding time, like almost all cats from the woodcat family.
Non-territorial, lacustrine synos (not riverine) are an agreeable suggestion. A school of dozen LSN (actually chances are you will get a Sorubim elongatus (max 1'), not Sorubim lima (max 2')), looks real nice too and they are not hiders.