Okay so I think it would make sense if spawn broodling actually spawned broodlings.
They could have included like two broodling plastics per zerg faction and when you use the spell you add one broodling plastic to the space the battle took place in. That would be the only change needed because broodlings could just use the minor values on combat cards and never qualify for major values. They would have either no support value or a support value of 1 and only ground attack capabilites.
Seems like something nice and easy to add to the game to make it more complete.