1st off your bichir MIGHT try eating those . By the belly of the bichir it seems to be a decent size compared to the vieja .
2nd off if you grow them as a group and then pick 1 and sell of the rest , max chances are that the lone one (if not the most dominant or aggressive of the group) will be very shy without tankmates . Imo if the final olan is to keep just 1 specimen , then try picking up the best one and keep it solo , it then can bond with the owner and be responsive . In a group after they establish the hierarchy their nature changes according to the position so you may end up with a fish that looks good but no personality .
I'd prefer a not so good looking fish with best personality .