I can't help with all of it but here are my thoughts. Multi shot is ok for hft but not ideal, as you have to remove the magazine between each lane as a safety precaution, so most people use single shot. However you can buy single shot adaptors for most rifles.
Secondly I have recently bought an hft 500 and absolutely love it! The stock fits great, it's a good weight, very very accurate and seems to get a lot of shots per fill.