there was a similar question a few months ago about sporeling and rush that basically concluded that sporeling might work different from other cards in the game
I’ll assume you meant Houndmaster Shaw (“Your other minions have Rush”) and not Houndmaster (“Battlecry: Give a friendly Beast +2/+2 and Taunt”).
Normally, when a minion transforms, the new minion is considered to be a new, just summoned minion. Which means that one would expect it to have summoning sickness, and that Rush or Charge would partially or completely exempt it from summoning sickness.
Based on what you describe, it seems as if Infectious Sporeling works differently. As if it change…