No.
The Aliens are feeble and weak as it is already. Being able to battle them in melee (and win) would turn them into simple zombie game cannon fodder.
If anything they should make the player even weaker in melee situations. Punching should just be cosmetic with zero melee impact and in order to escape the grip of an Alien you should need a team mate to help you. Acid splash damage should be potent on lower difficulty settings and so should acid puddles on the ground. Friendly fire should also be more of a concern no matter the setting.
Right now it's like the player is wandering through a shooting gallery packed with rabid dogs & monkeys + the ocational lumbering steroid freak who likes to use you for weight lifting. I get it that this is a fast-paced arcade-like shoot em up kind of game, but that doesn't mean that the Aliens need to be slapped around like trash.
For example, instead of having each Xeno die after a round or three, they could have it fall to the ground where it recuperate for three seconds. If you keep shooting at it while it's on the ground it will be weaker once it gets up again. Also, pistols and shotguns should only do damage up close, although shotguns should have a halting effect on the Aliens no matter the range. Incendiary weapons should not kill the Aliens but blind them and have them run around randomly (or even run away altogether), which makes it easier for the marines to finish them off with pulse rifles, smartguns, grenades etc.