One thing you should consider before you allow unmodified D+D races into your game is that D+D is balanced quite differently than Traveller d20.
D+D is very heavily balanced towards melee combat. This means that Str. is the most important characteristic. You'll note that the half-orc gets +2 str, but -2 on two other stats, whereas all the other races balance out on bonuses/penalties. This is because the designers decided that strength was the most important stat and any one stat alone wasn't enough to balance a bonus in it.
In T20 however, the abilities that allow munchkining are much different. Dex is the most important, as it governs ranged attacks (and therefore firearms and energy weapons). Con also seems to be more important than before as it determines lifeblood. This might balance out (almost) in the case of elves, but it turns a halfling into a combat machine.
The D+D race that would be the most effective in combat (in general) is the Strongheart Halfling from Forgotten Realms. +2 dex, -2 str (but who cares about the strength, you'll shoot anyone who gets even close). +1 BAB and +1 AC for being small, plus the dex bonus, plus a bonus feat and skills like humans get. The reduced damage from the requirement for smaller weapons is greatly reduced (Yeah, no ACR, but a laser carbine would still be ok, as would a nice snub SMG) and the only other disadvantage is slower movement.
I guess I'm trying to say that swapping races in from D+D is fine, just watch your characters to make sure nobody's going for an unbalanced ultramunchkin. Hopefully you don't have these problems in your group, but I've been playing with D+D munchkins for a while, so I tend to notice these kind of opportunities.
Sorry about the huge post.