legal but not that practical
purely on numbers, yes it could be done.
But the 150-hr f/w commercial has to include a bunch of specialized training (10 IR, 10 complex, etc) so you might have to add a bunch of expensive stuff to the 150 if what you already have is C150-intensive.
AND just the heli CFI may need more $$$$$ to pass insurance requirements, it seems to be a generality in USA that insurance requires 300 hrs heli before the school can use you as CFI. Building that extra 150 hrs in helis is very expensive ($30,000?).
PM or Email me with detailed time-in-type hours if you care to work out the needed additions.
Dave
a Chief Flight Instructor