One thing you should know about commando taunts is that the starting taunt (force hammer) is practically useless compared to the others two (graviton slam and back breaker). I'd suggest you respect out of hammer as soon as you unlock one one of the others. Graviton hits several targets at once and slows down target movement, and back breaker has higher damage than either of the other two, and combos with a knockdown skill for increased damage.

Cannons are very powerful against groups of enemies, the trick is usually getting the bad guys bunched up enough for cannon fire to be effective. The gravity well skill works well for that, and in theory so does singularity.
I wrote a guide covering everything I could think of regarding commandos, its stickyed at the top of this forum and there's a link in my signature. Feel free to check it out if you have any other questions or comments.