Hawks are active, tough fish and can be bossy but I think clowns of any species should be able to hold their own with no problem. I keep my pixy hawk with other semi-agressive fish: a damsel, saddleback clowns, sleeper gobies, a flagfin angel, neon dottyback and sixline wrasse. I'd avoid small, timid fish and (unless you're adding something more agressive than the hawk) add it to the tank last.