I think it would be too strong if you can attack your own units to create any combo. Some units are only limited by restrictive passives such as chastity/temperance/patience/reaver/merc, and some are 1-use like fireball or soulflare, but gluttony mechanics do not steal passives so gluttony could use these units to exploit their efficiency and bypass all negatives.

Even beyond this, simply combining a few choice units from your own army without much possibility of counterplay would be enough to break the game I think, and gluttony cost would need to be astronomical to match that extreme power. ex: eat a few cheap mages and a bat, but suddenly have something that should cost like 40, I don't think it could ever be balanced to build your own gluttony, lol.