Do you know what species of Giraffe Cat you have, some can reach 90cm. You could try Pimelodus ornatus, Pimelodus blochii, Synodontis sp, or Platydoras costatus, these would all stay <12'' and could live in a 100g for life.
Theres lots of pictus species that would do we'll in that tank a lima could work in your tank for a while but a larger tank would be nice mabye a few sun catfish
Ummm yeah surf around planetcatfish.com I like Pimelodidae, Doradidae and random others might wan't to give them a look go to Scientific names and look under those families