Mystus carcio - Tengara catfish grow to about 7-8 inches. two of them in a twenty gallon tank is already a high bioload, let alone the rest of your crew. in my opinion, i'd forget adding any more fish unless you upgrade the tank size.
The ongoing confusion of common names linked properly, or improperly to scientific names.
If you use Tengara Catfish on fishbase you come up with several options showing a length approximating 7". But, you do not discover a species with the scientific name utilized in your post.
Planet Catfish pops up with that sientific name for the common name Tengara Catfish. It does not route you to other possible species choices.
Many other sites--that are held in both high and low esteem use an 8" when discussing the fish you identify.
Obviously there is a lot of species identification confusion. I would personally go to a lot of the sites--examine the photography and make certain i have properly id'ed the fish. A difficult task.
ya there is conflicting info for just about every fish i research, which is why I come here to ask people who have kept these fish and can give me more accurate information. As for the tengara cats; the photo from planetcatfish matches my fish exactly so unless theres another species that looks identical; I think I have the right one. Here is the exact link to the fish I have http://www.planetcatfish.com/catelog/bagridae/mystus/326_5.PHP