Why don't you just use the USB mixer you have, and buy a USB midi interface for 40 bucks or so like
the M-Audio Midisport?
I think it's always better to have more channels just in case you need it...so maybe the mixer/interface is more than you need right now, but it is always good to have some flexibility and opportunity to expand a little in the future, if needed.
Anyways...I thought I would just throw that out there as a different option.
Good luck in your search!