I'm a little counter culture on that 100Ah battery (and many other things as well).
Batteries know ONE THING -- their voltage (and
current, I guess). They know nothing about anything else in the system. The 100Ah knows nothing about the 460Ah. It doesn't cause premature death, it doesn't cause over
charging.
Mixing battery sizes doesn't matter one
iota. As long as the chemistry and charge profile are the same, they don't care.
Differing length wires (and associated resistances) can cause a minor variation in discharging profiles, especially at high currents. But, on
boats where 99% of the day is running currents under 10% of the design currents, the voltage drop on the cable is extremely minimal. Remember, we design them for 3% drop (.36V) at full load, so at 10% of load, the drop is .036V. That means the
batteries are going to be well within .1V of each other. LFP has a very flat charge curve, and so .1 is a lot -- in the mid range. But when one of the batteries starts to get full (or empty), its voltage will start to climb (or drop) and the other battery will pick up the extra. Bottom line, in the mid-range, one or the other batteries may be ahead or behind on charge, but at under 1% or over 99%, they will automatically re-align. And you will NEVER get a situation where one is at 10% and the other is at LVC, or where one is at 90% and the other is at HVC. And, at the end of the day, when the batteries are full or empty, you will have essentially both batteries capacity (546Ah) available.
Since you absolutely MUST get to 100% charge periodically (SOC meters get out of whack over time, and if they never get to 100% and "reset" they can get WAY out of whack), every time you get to 100% they will be re-balanced and ready to provide
service again.
One thing to keep in mind in paralleling. ABYC assumes that in a parallel situation, loose connections can make all but one wire off line -- and so that 100Ah battery needs a wire matched to the main fuse, even if it normally provides a very low
current. Typical
boats use 4/0 for the main battery line, and so that means that 100Ah needs 4/0.