My min size for senegals would be a 40 gallon breeder. As for tank mates it would really depend on more in terms of width/girth than length. A wider bodied fish like gouramis, silver dollars, etc. would stand more of a chance compared to slender bodied fish like rosy reds. As for the dwarf gouramis, I have no experience with them so I'm not sure how they would fare with the senegals but a good alternative would be leopard ctenopomas.