You don't create those sprite collections - they are automatically created for you when you enable the system. You do need to create the textures though. Eg. if you use a sprite from the source texure "ABC.png" you will need "ABC@2x.png" and so on. Follow the instructions in that doc, and it'll create the 2x / 4x / 1x platforms for you with your override textures.
"How do you support an iOS build having both iPad and iPhone specific assets?"
Depends on what your game does. You could just use iPhone4/5 assets for iPad and zoom out to be pixel perfect. Or you might need custom assets completely. If you can deal with 1x, 2x and 4x multiples things are relatively easy. This gets considerably more tricky if you want odd multiples, but depends very much on your needs.
"How do you support a build that has both iOS and Android assets inside?"
Depends on your game. Can't androidsmall and androidbig be the same as 2x and 4x? If you can deal with that then you've just got 2-3 sets of assets. Job done
Or if you need other "platforms" you can add them but there isn't an automated way to deal with platforms. It
is doable manually or through a script though.