Battery Isolater concerns

Its my understanding that batts will only match voltage with mismatched batts.

Amps will draw or feed what they need. Least resistance...

Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Charge and discharge current will be shared, based on capacity. It is best if the batteries are of the same type and age. For example, avoid combining a sealed (gel or absorbed glass mat) battery with a flooded (conventional) battery because they have different charging setpoints. Broadly speaking, you can parallel batteries without problems, and the charge controller will look after them. Just make sure you give them plenty of charge. If the system tends to operate at less than a full state of charge, adding new batteries to old will probably just result in the old ones pulling the new ones down and everything getting sulphated.

If the 100 ah batts are the starting I wouldn't worry. My chevy truck charges the batts at 15 + volts at times.

And if you read the specs on your starting battery I believe it will say charge at the 14+ volts...