Mech hitting Neko isn't *that* big a deal. You do want tomake sure you know when she's behind or in front of you most of the time. Neko doesn't have a penalty for getting hit besides a few seconds of not being active. she also can't be hit by you if you're summoning her for something.
Personally I think C-Mech&Neko has some nice potential. 214 series is good for pressure since Mech can move about in tandem to Neko's attack and the startup is pretty fast. The A version of this projectile is slow enough so that you can use it as a meaty on the opponent's wakeup. You can even dash into them to push them along and right into Neko's hitbox.
F-Neko&Mech has some nice crossup potential with 214A or B on wakeup (depending on the position of Neco.) simply summon Neco and hold A (if you're using the A version) and then IAD over the opponent's head before releasing the button. You can cross up this way, mixing up between a proper air dash over the opponent or a quick 5C to cause Mech to drop down before the crossup. C-Neco&Mech can probably do this better since she has more time to move.