Other than not being the best way to go, is there any problem with mixing batteries with different aH together in your battery pack? I have access to a bunch of UPS batteries, all 12v, but will most likely not be able to get 3 together that have the same aH. I realize that mixing them will result in a lower combined aH than the highest one, but as long as they don't blow up, for now free and less mileage is better than great mileage that costs me $$.

You'll only get as many amp hours as the weakest one, if they are in series... and you'll risk ruining the weakest battery unless you make sure not to push it too close to its limit (maximum current or maximum amp hours)

If they are in parallel, you just add them up (but they must start at exactly the same voltage!)