2by10s are ok. use 1/2" spacers between the two every 16" or so. They can be pieces of plywood, backerboard, or drywall. On your 3 sistered 2by4s, have 2 of them go under and hold up each end of the beam. Make sure they sit on a 2by4 floor plate to help spread the load.
You can measure and cut your studs out. Put your end studs in, then lift your beam in place and pound the other studs in underneath at both ends. I'm not sure that 9' of an empty attic would sag enough that you need to brace it before hand.