fast charging batteries

20 amp.. That's not a fast charge, Even for a Group 24 that is not a fast charge. (in fact for a group 24 is it very close to the recommended charge rate)

Anything bigger well the bigger the battery the slower the charge as it were.
20 amps for a decent house battery system is approaching a trickle charge.

Here's my take...

Keep an eye on things voltage wise the first time round

No problem. I have had up to 3 chargers connected. Converter, Solar and Xantrex True Charge 2.

No trouble at all. Just slap it on and let it charge.