As to the boost switch question. It doesn't know what kind of batteries you have. It will gang what ever you have to the starter circuit. Your concern is whether or not your choice (design and number) of LFPO batteries are sufficient for a boost start without overtaxing your house batteries.
There are some considerations for both the number and max output per battery. The battery manufacturer should publish discharge information. (See attachment for my batteries as an example)
I have 3 of them and their combined PEAK output is 600 amps for anywhere between 5-10 seconds. There are lithium batteries with higher peak and/or continuous output but with higher prices.
PERSONALLY...If I ran into a relatively dead starter battery situation I get on shore or generator power before using the boost to start the engine. I would also force the charger into a bulk charge in an effort to give them a "shot". If not on shore power and I could not start the generator in the normal way I would use the boost button to start the generator and then force the bulk charge.
BTW, if you are concerned about whether your LFPO battery specs meet your starting needs, I would go straight to forcing a bulk charge and if needed starting the generator with the boost switch if not on shore power.
Don, Sandee & GSD Zeus. Guardian GSDs Gunny (7/11/15) & Thor (5/5/15)
2006 2015 DSDP 4320 4369
, FL Chassis, 2013 CR-V, Blue Ox Avail, SMI AF1.