its obv they can use fuel rods and brute shots, fuel rods are just rocket launchers but more rapid firing and less kick, and brute shots, those are less heavyweight of a weapon then the FR so...
the grav hammer? well its possible an ODST can use that, i mean, the main problem is how heavy they are, it may be some lightweight covenant resource, and as for the knockback, i guess it shouldnt really hurt that much more than it does a spartan, but noticeably more, but then again it doesnt really matter does it