It's very possible, I've done it on many occasions successfully!
Multiple output routing is contingent on your audio interface. The Focusrite Scarlett 18i8 (FS 18i8) is one of my interfaces and I've used MainStage to route both a pre-recorded metronome and audible cues to only the headphone outputs of this interface to avoid the audience hearing these musician friendly helpful tools (LOL). And with the FS 18i8, if the house is running mono.. I've also set up the 2nd main output of the FS 18i8 to act as a 3rd output increasing my number of routing options.
In addition to setting up the output routing sources within MainStage, the FS 18i8 has its own software mixer I have to use in addition to MainStage to successfully route things where I want them to go.
As for an "aggregate"... it will work, but with what you have. If you have an audio interface with multiple outputs, you should not need an aggregate (in my opinion). However, if your audio interface does not have multiple outputs, then an aggregate may come in handy in this scenario but it all comes down to how many outputs you need vs what you have available.
Which audio interface do you have?