No question that all mode infantry is possible and that acceptable performance can be attained in all modes. The problem is that IMO, acceptable is okay but I prefer excellent performance if it's attainable -- and that is if units concentrate on their core tasks and doing them really well instead of getting to a fair performance level in many tasks.

For mechanized infantry with vehicle size constraints, 9 men is fine -- because the vehicle itself adds combat power and capabilities that compensate for the missing four men in the walking or airmobile infantry squad (where there is no added combat power).