Newbie Question: Can u invade planets that you cannot colonize?

Yes, you just won't have any production there at all until you do get that research by some means.

The planet will show up on your colony screen, but all production has the helpful word "NEVER", indicating (as Kyro says) that you'll get nothing until you research that colonization tech.

Of course, you may get the tech when you invade - kind of a welcome-to-the-neighborhood present.

Hydro

And of course, you can still rush-buy projects. As long as they're not factories, you'll be fine.

Won't morale be really really poor on such planets? I'd imagine a Dread Lord-type situation where only 20m is supported.

You would wonder why, seeing as how the terraforming has long since been done The islands aren't sinking back into the sea or anything.

I agree with you. Once the planet has been colonized, it should OK for all civs to live there. Thats the advantage of going to war. You want them to do all the work so you can just kill everyone and steal the planet from them.

A hint: You could save the game before you invade, then keep invading until you get the colonization tech from them. It's just the long way of doing what the game should have been designed to do anyways. The logic of the creative design of this game borders on the stupid sometimes.

well, logically they should be able to use the captured facilities,,, only the union rep won't allow work to proceed without the proper technology!?!? lol

Heheh. This whole environment and related techs sounds like a rush job, and ultimately shows an incredible lack of creativity. This could have been a very interesting new feature, but it turned out to be just another source of the good old cheese.   
The thing with the aquatic environment is that it shouldn't really be a special environment - these should be the ones that required a domed colony. Radioactive is pretty dumb too. Researching the techs should be something like how to build the corresponding domes, and environmental suits, and the like, not creating islands - but then again, it's in the line of some of the (ridiculous) invasion tactics. So in order to invade such planets, you should have the techs. The exploitive factor (the AI is not really capable of defending against a human player) is high - but then some people lke that.
Opinion, since the devs concentrate solely on adding new stuff, it'd be great if they did it in a mature fashion, because if it's marginally working, that's the way it'll stay.