Battle Born was not very explicit about it in the Zoom meeting last week but in the long run it is very much advisable to use the 24 volt units instead of the 12 volt units.
Using 24 volt units makes sure you will balance cells across 24 volts instead of 2 x 12 volt leaving a potential unbalance between the two 12 volt banks.
For capacity the answer is simple, as much as you wallet can last. You will always find ways to spend the available capacity in the end, so more is better.
For your chargers you will need to investigate if you can set them to the exact same charging parameters. That may be difficult.
Also think about your charging regime. Again Battle Born was not very explicit about this but if you boat is connected to shore power for extended periods, you should lower your float voltage to reduce the state of charge to 70-80%. This can be difficult to achieve if you have several charge sources as this means adjusting it very precisely, up to tenths of volts accurate.
I did not like the way Battle Born played down this issue but Lithium batteries do not like to be kept at 100% charge level for extended periods. Even Tesla has extended logic in their battery management to handle this issue. So when Battle Born claims it has superior cells that do not suffer from this problem, I’m skeptical.
Just my 2 cents....