As for what gestalts do in an internal politics DLC?
Make 'gestalt' a 2 rather than 3 ethic pick. That allows you to be a gestalt with a particular interest and opens up more civic possibilities (a Xenophile hivemind that is so interested in other races it gently infiltrates them and takes over, before getting 'lonely' and starting with another civ say?) and restyle their factions as moods or impulses. That would be a better hook point for it and give them some versitality.
I wouldn't use the standard ethics, I'd give them a set of "personalities" that governed how they interact with the rest of the universe. Things like Cooperative, Domineering, Inquisitive, Hostile, etc. You would pick one at the start and be able to change it more freely than a standard empire can change ethics, because you're just changing your mind.