I've got a 6906 that I've been using 5900 series mags with the grip ext. I've seen that mec-ger offers a 17 round mag for the 5900 series guns. Will those mags work with the grip ext's in my 6906?

I have the mags you mention and I don't think the mec-gar mag will work with the +3 adapter. The mags are made different at the base plate preventing the adapter to mount.

So the 5904 14rd mag will work in the 6906? I would appreciate a quick education on mag interchangeability of 3rd gen 9mm autos.

You need the smaller/thinner base plate on the 15-round mags to make the adapters work. The thicker plastic ones will not allow for the attachment of the grip adapter to the mag. I have found that the mags don't stick out all that far, and that the adapter makes slamming one of them home a scary moment, and a "tea with the queen" grip is required to avoid blood blisters.
I carry the 6906 with a 12 round mag in it, and stick the 15 or 17 round mags in a carrier.