I'm currently playing a first level Swordsage/Fighter combo (mostly for the BAB and feat), and was thinking of going Psy Warrior for my second level for more versatility. Currently I use a THW, waiting for more gold so I can upgrade to a Large one using Monkey Grip. I think it would be hilarious to use Expansion and SS maneuvers with a Huge Great Axe. Any suggestions or improvements that could be made? Sorry if this is posted in the wrong forum.

Honestly you would do better by going into Cleric for the rest of your levels along with Swordsage. Two levels of Fighter are plenty and Cleric will give you the ability to buff yourself with spells until you have a better to hit bonus than a fighter.

What feat would you recommend I take for my second level of Fighter?

3.5 has a truckload of Feats for you to choose from. You already mentioned Monkey Grip but honestly it depends heavily on what sort of character you are creating. Another essential feat would be Adaptive Style from the Book of Nine Swords which would let you change your blade magic loadout on a whim. Extra Readied Maneuver from the same book can also prove to be a simple but useful feat. When in doubt, just buy Improved Initiative or any of the cleave series.

Can you take Adaptive Style as a Fighter bonus feat?

Not that I recall no, but you should consider it for your 3rd level feat if not going for Divine Metamagic.

I have no idea how DMM works lol.