yes there are quite a few channa punctata,melasoma,pleurothalma,aurantimaculata,asiatica or parachanna africana,obscura or insignis,this is off the top of my head so there are prob loads of others that may be suggested by people with a larger capacity for remembering names than me!!!
yes they will all do well in a 6x2x2 but obviously not all together!!! there are other species also that you could consider,have a look on snakeheads.org