| Kazaan |
That actually sounds pretty interesting. There are a number of Eidolon traits centered around making an "undead" Eidolon, too. It won't "actually" be an Undead (it still counts as an Outsider) but it gets treated as Undead for several purposes. So you can have your hoards of Skeletons, as well as an "Undead Champion" Eidolon. Your GM might question whether Skeleton Summoner will work for the MS's Summoning Mastery ability, so, if it comes up, point out that "ability to cast <specific spell>" includes casting it as a SLA as per this FAQ. So, your Summoning Mastery both qualifies as a prereq for Skeleton Summoner, and the feat will affect it in turn. Also, note that Summon Mastery calls out that, save for the explicitly stated changes, it otherwise functions as the normal Summoner's ability Summon Monster. That means it still gains access to higher levels of Summon Monster and you aren't just stuck at Summon Monster I forever (I hadn't noticed that at first and thought it was stuck at tier I until I read it more closely).
But the real issue is that Summoners can slow down the game and also take a lot of logistics to manage, especially with large hoard-type summoners. I've played with a GM that hated summoning, even to the point that he wasn't to keen on any class that had a built-in "buddy" (companion, familiar, etc). Are you going to be able to handle the logistics of calling up 2-6 Skeletons at a time? Will your group be OK with having to wait around while you handle 3-8 turns per round, compared to their 1-2? It'll kind of be like having two separate GMs, each handling their own swarm of monsters and maybe almost doubling, or at least increasing by half, the number of turns handled per round of combat. Don't exclusively focus on how you "could" do it; spare some thought for whether you "should" do it.
Edit: You may also want to look into Harrowed Summoning to further boost your Skeletons. Even with Augmented Summoning, you only end up with about a 5.5% chance to gain no stat bonus at all and only a 22.2% chance to halve the duration of the summoning (and a 22.2% chance to double it).