The 7649 Mini-Add-A-Battery has a continuous rating of 65 amps and as far as I can tell your engine has a 50 amp alternator. You might want to check that in the owner’s manual though. As long as the alternator output is 65 amps or less this is the right product.
Your question about wiring depends on exactly how this is setup but to use the full 65 amps of capability you need to have at least 8 awg wire. I am assuming the start battery is near the engine. Note that the two battery banks must share the same negative also so the negative must also be at least 8 awg. In order to use the house battery to assist starting if the start battery went flat the wire would have to be quite a bit bigger because of voltage drop, I would suggest 4 awg