After playing around with a few different combos, it is defiantly possible to mix them, but in my opinion I think it is best not to do so. Tangs are not from Lake Malawi, they are from Lake Tanganyika. Depending on your foot print on your 240, you could do a male peacock show tank, a dwarf tang tank, or maybe a small mixed mbuna tank- yellows, rusty, snow white etc. But in all honesty, your 240 is on the smaller size for Africans- not impossible, but does throw up a bit more issues.