Different mAh pairing, is it possible?

I have the first board I built and it is a 6s setup. The original 2x 3s 5ah 30c didn’t give enough range so I replaced them with 2x 3s 8ah 30c. Could I add the original batteries to the current batteries to make a 6s2p?

yes - but you will likely kill the 5ah batteries if you leave them connected in parallel. Better to throw them in a backpack and change them should you need to. I had 2 6s 5ah batteries in a turnigy zip locked battery case - when one got low, I plugged the other in and headed home.

Unfortunately I have the batteries in a bolted on enclosure. No worries though. Was just curious if it would work with them together.