Accell is only refering to the processing cards....and as far as I know there's no "Accell" versions of plug-in's..
I could be wrong though.
There are more and more plugins that are being written specifically for Accell chips (particullarly from Digi).... But most of the plugins do need to be updated to work on the Accell chips, or they will only open up on the original (HD) chips.
Yes, you should be able to use the CS just fine, but if it hasn't been updated or wasn't written to work on the Accell chips, it will only work on the first card.
Actually the first two chips on the Accel card are "Presto" which are identical to the HD Process and HD core card. The rest of the chips on the Accel card are "56321" which are the high performance Accel chips. Any plug-in that is HD compatible will run on Accel. But plug-ins that are optimized for Accel will only run on the "56321" chips.