This solution – one airbag that you can swap between different RST clobber – has got to be ideal. The RST stuff explicitly designed for the InMotion system has elastic panels in the right place to let the airbag inflate properly. My 'Tech-Air compatible' jacket is similar, and it means I can wear an excellent fitting jacket, with the confidence that the airbag can inflate fully, yet the armour can't move out of place.
I've seen others buy a jacket that's bigger than usual to allow for airbag inflation. But I reckon it means a tradeoff between increased safety from the airbag versus a loss of protection from a slightly loose fit (the material could fold over and abrade more easily, armour could move in crash conditions).