@ AQUAMONSTER - You have to realize however that a) lungfish are very flexible and b) a 2 foot lungfish is far from adult. Also, yes, there are people with 2'+ fish in 125 gallon tanks. And in my opinion, it is wrong. "He did it as well" is a bad excuse for doing things.
@ OP: I (and many others), would consider a 6' x 30" the minimal footprint for a ray, based on the fact that they will eventually grow to be at least 1' across and will have a tail to add to that.
In the end, if you feel it is ok (and are able to keep up with the maintenance), you can cram any size of fish in any size tank. I however feel that a fish should at the very least be able to turn around comfortably. Just a personal preference. Keep in mind that with overstocking comes trouble, such as oxygen depletion, higher nitrate levels, stress, etc.
As for catfish, there are many that could work. I personally find plecos highly interesting and that tank could house a few panaques comfortably or alternatively, you could stock it with meat eating plecos such as pseudacanthicus species or scobiancistrus aureatus. Alternatively, if you like catfish that swim a bit more, you could look at pimeloids, such as sorubim lima, Aguarunichthys torosus, pimelodus ornatus (or brachyplatystoma tigrinum by aquamonster's logic of cramming things into tight spaces). Since I sense that you are not interested in smaller fish, you might also look at the Auchenipteridae family. You could keep a few Asterophysus batrachus, but this would rule out most tank mates and leave the upper parts of the water column empty. I personally love the shape of ageneiosus species.