So im thinking about a summoner build for my upcoming game, it'll be starting as a 15th level character, and im looking into a mounted build. Huge size, with flight. My question is im looking into trample and pounce, would the eidolon be able to charge, trample over an enemy or two and then full attack at the end of the charge while I attack with my lance (power attack, spirited charge, vital strike)?
No. Trample is a special full-round action that lets you move through a creature's square and attempt to do trample damage. It is not a charge nor is it an overrun. It uses the rules from overrun for the framework of action options, but is not actually an overrun move.
As a full-round action, a creature with the trample ability can attempt to overrun any creature that is at least one size category Smaller than itself. This works just like the overrun combat maneuver, but the trampling creature does not need to make a check, it merely has to move over opponents in its path...