Assuming that the tank is only 29 litres then most catfish won't be suitable, even the smallest L no. plecs (including Zebras) require a minimum of 60 litres. Still Corydoras pygmaeus might be worth looking into.

There are quite a few species that can be mixed with Siamese fighting fish (Betta splendens), small tetras, certain barbs (i.e cherries and pentazonas), small rasboras and danios can be mixed. Fish with long fins (i.e guppies) and other anabantoids should be avoided.