forced air duct questions

I am trying to finish out my basement and need to move some air ducts.

Is there an easy rule about how many registers can be on a certain size duct?

Right now I have a single rectangular duct that is about 8" x 14" and there are 6 registers fed off of it. 5 of those are on 6" round ducts, 1 of them is just a register in the duct itself applying air to the basement.

This main duct runs right down the middle of the area I want finished. The good news is there is a 7" tall beam in the general area. My thought is to make 2 smaller ducts, 1 on each side of the beam and use those to feed the 6 registers, as well as maybe 1-3 more registers for the basement. There is also a possiblity that 2 of the 6" sub-ducts can be moved to an independent run to another part of the basement.

I guess the question is, if I reduce the 8 x 14 duct into two seperate ducts, can i just make them each 7x8" and reduce the # of registers on each?

Also, if these are metal ducts what an I do to reduce condesation on them? I'm guessing I just need to insulate them well, but what with? I don't want them dripping onto a finished ceiling.

Thanks...