[whswhs]

That seems solvable. Being able to summon the large slime is the Allies advantage, bought as an ability of a power. So it can take power modifiers. So you let it have the Cosmic modifier, +50%, and that lets it violate one of the standard rules. Now you let it violate the rule that an Ally can't have Allies. Now you can give it the swarm as its own Ally. This doesn't give you the ability to command the swarm yourself; you have to work through the primary slime.

Or maybe you could define it as the "half-Cosmic" version of Weird, for +25%, though I think you would still have to pay +50% for the ability to violate a standard rule.