Multiple Batteries in Parallel

Yes, but the main thing is they be in similar "condition", which means past usage and care of each one. Not good to bank a battery in poor condition with a good one, or you soon have two poor ones.

You can parallel different "size" batteries of similar condition, no problem. Eg an 80 AH with a 100 AH to get a 180 AH bank. They will draw down together (if balanced) so that at 50% one will be 40AH and the other 50 AH. They will recharge together and get to "full" at the same time also.

It is hard to balance several batteries in parallel. One way is to group them in smaller banks and then bank two groups. More wires but so what? With four batts you can make two pairs and then parallel the pairs. Then put your positive load/charge wire on one pair and the neg on the other pair.

Assuming they're of similar chemistries/design and ages and conditions, it should be okay. Batteries in parallel are never ideal but often times necessary. The total capacity will be the sum of the various battery capacities, basically.

Your battery pack will be as good as your smallest battery