Wow!!! Kudos to you

CDing twins will save a LOT of trash from the landfill!
So first of all, some may tell you not to invest in a prefold only stash before you try more, but I will not tell you that

I love prefolds and you will get tons of practice to get your folding just right! I know people love both Imagines and GMDs, I personally used GMD and love them but Imagines are indeed cheaper and not bad quality. I don't think either one would be a bad choice.
I will say though, that covers are different... Some covers just fit better on some babies... I would get maybe 8 of a variety of covers and then order more of what you like best. You might like some at night, some for rashy times, some when you need total bulletproofness, one kind fits best one week, one the next, etc. 16 xsmall thirsties just sounds like a bit much to me. 16 is a great though, it is always good to have plenty of covers, and it will be no problem if your folding isn't perfect and you get a lot of poopy covers!
As far as numbers, 4 dozen nb prefolds sounds about right for newborn twins. You will probably not be able to fit much more than 24-30 diapers in your washer at one time anyway. You want enough to fit in the washer, plus enough to use while the wash is being done, plus a healthy safety margin
Good luck mama and congrats!!!!