The guy who put a Pak 68/3 accelerator board into a STacy for the very first time, dubbed it "Super STacy". I'm good with that. I'll just leave "mini STacy and "STacy mini" right where it's at.
You're not wrong about pricing. Like so much retro stuff these days, the costs are amazing. I noticed at SouthernAmis that even in the 8bit world, the MIO's are going for $350-600.00 on Ebay.
That's actually right - the internal ACSI port is the same on the Mega STe and STacy - they share the same controller board. Interestingly enough, some STacy's have a place on the motherboard, with an external cover, for the Mega ST port as well. My STacy has the external cover and the place on the MB for the connector but it's not actually there. It's mentioned briefly by John Jainschigg here:
https://www.atarimania.com/documents/atari-stacy-Atari_Explorer_September_October_1989.pdf