Eupterus and decorus are high good synodontis for those conditions and tank size. Avoid ones like petricola or multiply catfis since they prefer ph over 8.0 but the two I mentioned do fine at that.
If considering dwarf giraffe cats, size depends on the genus, notoglanidium (formerly anaspidoglanis) species will only grow to around 7"or 8" but most parauchenoglanis species will grow to 17" or so. All are omnivorous bottom feeders by nature but will eat very small fish if they can but are not really considered predators and do fine on pellets and worms.