Not sure you can combine burrowing with Mounted combat. Which could end up being an issue.
A summoner is not a warrior, so mounted combat in general is probably not the best use of your feats.
Spirited Charge lets you do double damage when your mount charges the enemy and then you make an attack with like a lance or something. It stacks with Lance (so you could do 1d6+whatever times 3) but move-hit-move is part of Ride-by Attack which is its own hassle.
If you're married to the idea of a mounted gnome warrior I recommend Paladin or Ranger, with your special mount being a Giant Gecko because it is fast and can run up sheer walls. You could also do summoner/(warrior class of choice) but then you would have to figure out where to strike the balance of which class to take, and you would lose a favored class bonus half the time.
If you just want to normally be riding your eidolon, I'm not sure you even NEED the Mount evolution. You just strap on a saddle and ride, hopping off when things get combative.
