GENERALLY, you will want to stack your Rooks along open files to control the whole file. Which is why in this case Rexd2 is best, stacking the rooks along the d file, which can be used to infiltrate into blacks set up, this may eventually make it easier for you to target your opponent's weakness in the future (Eg. His b pawn in the long run)
What piece is best to take with / rook position
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Rexd2 is the most natural move in the position. Rooks work best on an open file. Also, R1xd2 leaves your back rank unprotected. That's a bad idea. Not only because you could blunder a checkmate, but also because it gives your opponent time. Basically their next move could be to play 26. ... Rc8. This is a developing move, making the rook much more active than before, while you'd have to spend a tempo on defending the checkmate.
Generally you'll want to control the open file. The dream for your rooks would be to invade the 7th rank (with white) as there are many yummy pawns to snack on. So here after 26. Rexd2, Bbd6, you could play 27. Rd7 hoping he defends with 27. ... Rb8, cause then you're making his pieces very passive, while you're as active as possible. After that you could best take some time out to get that back rank problem solved, by making a pawn move to create some air or by moving Kf1 (the king is an important piece to keep active in an endgame).
In this position, what is the best piece to take the queen with and why?
Is it best for the rooks to be standing behind each other or beside each other, as a general rule?
Maybe take with the bishop is best, threatening a5 (although protected) and alowing f4 on the next move protecting the advanced e pawn?