it's better if the batteries are rated the same.
No it is not, as related to charging as second bank with a combiner or B2B charger or via a BOTH switch.. It certainly won't hurt but it is absolutely not a necessity. Just because you have said it, and stated it as a fact, does not make it so..
Once you start adding external charging sources IE solar, it only takes a short time for the start battery to get cooked if not switched or correctly isolated.
Again, not true, I add these systems nearly daily. As a marine electrical systems specialist who actually specializes in charging systems, alternator, wind solar etc. I can assure you this is misguided.... Don't worry you would not be the first to assume things like this..
There's been many a posting here ( perhaps hundreds, even thousands) over the years with battery/charging issues suggesting there's obvious issues with the way many boats are wired.
There are postings all over the net of people with battery issues. I deal with boat owners multiple times per day and the one common denominator is that people, including many "professionals" simply don't understand them. This is plain and simple to see, based on this thread alone.. To assume
these failures are because the batteries don't have a similar Ah rating is simply incorrect. Most battery issues I see are due to user error, abuse, lack of proper care, poor chargers, excessively deep cycling, wrong charging voltage settings for the batteries being charged, wiring errors & chronic under charging. Chronic under charging is the number one reason I see for short battery life. A lack of understanding of the state of charge, resulting in excessively deep cycles, would be #2 and they tend to go hand in hand.
I have never seen a WET start battery "cooked" by being combined with other "WET" batteries because it can't physically happen if the other bank is not ALSO being cooked at the same time. In almost all cases the start battery lasts longer than the house bank because it is rarely cycled.
I just replaced a bank of Crown 6V batts last week that was 7 years old (this would have been season #8). They were the same age as the 60Ah starting battery and purchased at the same time (not deep cycle or dual purpose but a thin plate "starting") battery. The start battery is still testing within the safe range and is still usable. The start and Crown 6V's have been "combined" with a West Marine version of the Yandina combiner under "combined" circumstances for 7 years. Cooked? Really? Not in the real world..
Simply put one battery bank can not be "over charged" if they share similar charging voltage parameters... The Ah capacity of the other bank simply does not matter.
Keeping the ratings the same prevents over charging of the lesser battery.
Keeping the Ah ratings the same, as you've stated they need to be, does no such thing. Having batteries that are to be "combined" with similar suggested charging voltages is a good idea but the Ah capacity of the start battery means nothing.
There are some general guidelines that are good to follow when charge combining or using a battery to battery charger.
*Avoid mixing GEL & WET or GEL & AGM as one can be over voltage or the other will be under voltage. The Sterling B2B charger allows mixing GEL with other types...
*With combiners, ACR / VSR or a BOTH position try to avoid mixing battery types eg: WET and AGM.
With battery to battery chargers (B2B's) such as an Echo Charger, Duo Charge or Sterling B2B you can mix WET and AGM but unless using the Sterling I would avoid mixing GEL with other types.