Since you prefer not getting any .2s, then use the .25s. The .12g BBs are basically for low-end spring guns, and barrel-free grenades only. Don't quote me on this, but I think the hop-up is preset to .25g BBs on actual AEPs; if I had to guess, the ideal weight would either be .25 or .23. I could be wrong, but it couldn't hurt to use your .25s.
The real question however is if it's an actual AEP or an airsoft pistol commonly mistaken to be an AEP, the EBB. Ask yourself this: Does the slide move when the gun is fired? If it does, then go ahead and use your .12g BBs, because electric blow-backs aren't worth trying to care for. Although they are technically AEPs, which I believe stands for "Automatic Electric Pistol", they do not perform anywhere near true AEPs, such as the CYMA CM030 Glock 18c or the Tokyo Marui line of AEPs.
But of course, you probably already knew that, since you own a bag of .25g BBs.