Ficus benjamina puts out aerial roots very readily in humid conditions. Wrapping plastic around a stem or branch produces very humid conditions, you shouldn't have any problems.

I've air-layered them sev times, but smaller branches.

your roots are very small for the leaf mass - it might start dropping leaves. i'd put a large dry-cleaners bag on it to keep it tented for like 6 weeks or so until it grows enough roots. and light trimming should be done too.

I take it you still have a few long branches left on the original plant?

the cut stubs should resprout quite fast and then you can air-layer the other long branches..etc so that eventually you could have sort of a dwarf-tree look: fat trunks sprouting mops of leaves.

but all of them needs to be pruned regularly (at least twice a year) and that is besides pinching the tips - otherwise they'll be sparse and leggy.