Doctrine even showed that the Firefly would be kept back until it was needed to deal with German tanks.
No arguement that they would try to keep them focused on enemy armor or the
possibility of enemy armor but war is war and it would not be possible under all circumstances to do that. Fireflys are an integral part of a Sherman troop in Sherman units or deployed as a troop in a Cromwell squadron in Cromwell units. On the line of advance there are no guarantees as to whats around the next hill, hedgegrow, village, ect. You cant sit around all day waiting for a textbook engagement when your orders are to get moving. Also enemy counter attacks have a nasty habit of getting in the way of such niceties.
As for the exact quantity of HE rounds sources vary as they do ( nothing like a good source skirmish

) but as in all things real life would rule as king in the field and not some manual or doctrine as to what was listed as to quantity. I doubt many AFVs in the field in NW Europe in 1944 had everything just so; like infanteers, tankers would do what they felt they needed to to fight and to live another day.
An interesting discussion in any event.
...Oh +1 to the M10 in the game, it was a major Allied AFV in WW2 and deserves a place at some point.