If a smaller gar gets spooked in a big tank there is much more room for it to build up speed when speeding across the tank, causing a collision with the glass to be much more dangerous than if it were in a smaller tank. These can result in broken spines/backs/snouts and what have you.
it will be about one foot long is this size safe? also instead of a catfish how does a clown knife sound? will both fish have enough room to swim when fullgrown. I hate over crowding my fish and usually understock on purpose.
With the african/asian species of knife fish I have not had issues when I had them with gars in the past, provided the fish were of similar size. With a clown knife you have to watch out as some clowns can be quite aggressive. It's a combo that can work but you would really have to keep an eye on.
The african/asian species that I have housed with gars in the past were clown knives (regular, and golden), royal clown knife and african brown knives.
Oh... nope... they can't swallow 99% of tankmates because their jaws allow for a very small gape. Even big gars can't eat many small fish. That does not stop them from killing them though.