set of 12s- which tunings will they take?

Aside from standard, I play 12s in Open D, Open G, Open G minor, Open E, once in a while Open D minor, and have played in Open A. (I don't use that one much.) Open E (or A) is easier to play because of the higher string tension, but if you develop a good feel for where the slide stops by "muscle memory," you don't really need string tension to support the slide. A light slide helps.

Thank you, mate. :)

You're welcome. Merry Christmas!

Those will be fairly firm at open E and just slightly slack for open D. It depends on your touch; I like very firm strings for slide to avoid fret clackin'.

Is there any way I could capo to open G to get the same tension?

Merry Christmas to all!

Nope. Capoing doesn't increase string tension. It makes the open string pitch go up by shortening the strings, same as if you were fretting all the strings at that fret.

It should have been able to take DADGAD, right?

Well- I was coming back up, got to Eb, then the high E snapped. See other thread "First time changing strings- should I go for it?" for precise details. :)

It didn't snap because of the tuning. Where'd it snap? A common problem with going to larger strings is the string binding in the too-tight nut slot and breaking above the nut. (I don't remember whether the 12s are a step up or down in your string gauging.)

You got it in one, Ricochet. Above the nut.

A problem I've run into often. What you've got to do is to enlarge the nut slot a little so the string can slide easily in it, and lubricate it. Easiest way to loosen the slot is by taking a string of the new, larger gauge (new or used) and "sawing" it back and forth in the slot. Angle the string toward the peg it goes on above the nut, so it'll smooth down the corner the string tends to hang on. The wound ones really act as a file. The smoother plain strings have enough surface roughness to wear the slot to fit, they just take a little more work. Graphite is usually recommended for lubing the slots, and can be applied with a pencil. I've found anhydrous lanolin (available through pharmacies, candle and soap making suppliers) is a great nut lube.

Nope. Capoing doesn't increase string tension. It makes the open string pitch go up by shortening the strings, same as if you were fretting all the strings at that fret.
...but you can decrease tension with a capo - tuning lower than the tuning you want will decrease the tension, then capoing will bring the pitch back up to where you want it. It's no substitute for doing it properly with suitable string gauges etc., but it's good enough for a quick fix :)