Huntsman,
Your idea is a good one, and I heartily endorse it
The 120 grain bullets give noticeably less recoil kick than the standard military size 140 grains.
Recoil will be somewhat like the .243 Winchester using 120 grain bullets. Moderate, and comfortable shooting for youngsters.
Any of the medium to slow rifle propellants give good velocity in this cartridge ( IMR 3031, I|MR 4046, IMR 4895, and others).
For your carbine-length 6.5x55, I would use reloading manual starting load data and work up from there. emphasis on accuracy, and not maximum velocity.
Your twist rate should be fine for most everything from light-bullet varmit loads to the full-power military bullets in that carbine-length barrel. I shoot the M38 Swedish Mauser with fast military-twist and it stabilizes just fine with 120-160 grain bullets.
Good luck shooting AND hunting!
HTH
John