The no two weapon clause pretty much means no TWF.

If your BAB is high, you get multiple grapple actions. If you attack with a natural weapon, that counts as one of them. You still can't violate the "one attack per natural weapon" clause, though. So if you have 2 claws and a bite, but you only have a +6 BAB, you can make any 2 of the 3. If you have a BAB of +16, you can make all 3 and one other grapple action.