I think it is a good move to have granted the whole player base access to a free S Class, since you decided to go ahead with it anyway.
Having said that, have you thought about what half the player base having access to a Collateral Damage S class will do to hordes? It will completely negate 90% of strategies, due to the fact that most rely on swarming.
The problem is that due to the insanely huge gap between 6 stars and 7 stars, sorry, I mean S Class, you are now in a corner with hordes that I don’t see a way out of until everyone has a decent enough S class roster (well there is one option but I’ll come to that). If you don’t upgrade the Walkers, it will become exceedingly difficult for anybody to score points, especially with the random cards aspect (that I like btw, you just need to double or triple the starting cards), even if just fighting 1 S class. However, if you do upgrade the walkers to be able to take out S class, people will curb stomp almost every team, as very very few have multiple S Classes.
The only way I see around that is restricting hordes to no S class toons. Otherwise it will become the most unbalanced aspect of an already lopsided game.