If we're changing infiltrators into spymasters (which I think is an excellent idea, and Tom quite liked it as well) as part of the plan to create plausible deniability then having them use different troops from everyone else seems contradictory.
Tom does like the spymaster concept, and it neatly dovetails into another idea he had recently.
Irregular would be a mode just like Regular Army or Vanguard, so that shouldn't interfere with plausible deniability, whereas a restriction to Special Forces would. Admittedly Special Forces would probably work quite well as Irregulars but that would be a decision for the player to make.
One thing it's definitely not intended to be though is a means of turning infiltrators into Rambo-like characters, more the sort of partisan and guerrilla leaders which every age has thrown up.
However I'd be happy with the existing command options for infiltrators if other aspects of the spymaster potential were beefed up
