If so, did everything play nice together?

Unless I'm mistaken, the EPC9 is the Aero blowback 9mm. I'm not familiar with the Stearn Defense bolt, but assume it's like any other 9mm bolt cut fir glock and colt mags.

Aero actually sells Endomags for use with their 9mm rigs, so I guess you can say they're supposed to work.

My experience with Endomags has been very positive. My AR9 has gone through many iterations ranging from using glock mags to colt mags and now, Endomags.

I've found the Endomags to be the most reliable. Mine have fed all ball ammunition and most hollow point ammunition, which neither the glock nor colt mags would do reliably.

There are a couple of caveats. First, my AR9 won't feed reliably when it's upside-down, no matter the magazine. The Endo mags do very well with the AR9 rotated 90 degrees to either side, which I had some problems with using other mags. The second thing is that the ejector on the blowback models of Endomags can get damaged which totally screws them up. Endo will replace them.

My current AR9 is a built around a RDB setup and I use the ejectorless endomags. These have been rock solid.

Have the new Exomags on order so we/ll see. Mean Arms has no ETA for them they will start shipping, though. Better not be months from now

No personal experience, but from what I see of the photos of the Aero EPC9 upper and the Exomag/Endomag, there is an interference due to the LRBHO trip in the upper.  As is, the Exomag/Endomag won't seat.
I suggest that you contact the respective manufacturers.
Good luck.
MHO, YMMV, etc.

Thank you. I just asked Mean Arms if the mags will work.

From Mean...

Yes the ExoMags will work in the Aero EPC9 upper. Because this upper is designed to work with Glock mags you will need to remove (and reinstall) the plate on the left side of the upper and remove the LRBHO bar. Be sure to keep the linkage bar in a safe place in case you want to use a dedicated 9mm lower instead of an AR-15 lower. The same is true for our EndoMags as well as the ExoMag.

Thanks for posting the MEAN response. My suspicion of an interference between the parts as sold was confirmed, and MEAN provided you with what I assume is their preferred solution to the problem.
Good luck.
MHO, YMMV, etc.

You are welcome & thank you. Too bad they have no ETA on the Exomags yet

I can verify the endomags will not work with the EPC9 upper due as was stated to the LRBHO interferes with it. Luckily I only ordered one to see if it would work and I salvaged that one to use in my Banshee by cutting the ejector off with an exacto knife . They work great in my CMMG Banshee.

They work great with the BRN-9.