Not arguing the rules here, just the logic behind them. For the record, im fine with the changes, however, on Hoth, those troopers most certainly could have fired until all of their weapons melted before even scratching that AT-AT. The typical AT-AT Walker carried armor over a foot thick, of durasteel, which has an estimated (has to be estimated as the shit doesnt really exist) hardness and density near that of compressed irons inside of an asteroid.
Now, using a small asteroid as the comparison, it has been shown that an asteroid roughly 450 feet in diameter (roughly that of an AT-AT) would take about 50 million joules of energy to completely destroy said asteroid such as the ISDs did when moving through the asteroid belt on ESB. (Depending on how much you trust scientists math) Now, thats total vaporization, to simply break it up, scientists calculate it would take somewhere in the vicinity of 642,000 joules of energy. Your average 1 Megaton thermonuclear device only releases an equivalent to 450,000 joules of energy. So in essence, we wouldnt even have the technology with our most powerful weapons to hurt it.
Now as to your standard starwars E-11 repeating blaster, one of the most powerful personal anti-infantry weapons in star wars, only carries about 10 joules of energy per shot, and the extreme cold temp on Hoth, and the large area for dispersal on the AT-ATs armor, even continued fire on the vehicle would be absorbed and cooled before being able to cause any sort of damage. So realistically, yes, even 1000 squads of troopers would be unable to make that thing even flinch using normal anti-infantry weaponry.
Edit: It sucks to be smart, it takes all the fun out of movies.
Edit Edit: Also, this is why I say dont refer to video games when trying to make a realistic pen&paper RPG. In a video game, of course you hand held blaster has to damage the big AT-AT, otherwise people would complain the game is impossible to beat and not buy it. People are whiners now days.
Edit Edit Edit: Although, I thought the system we had was pretty nice as it forced you to use specialized troops to go after most heavy vehicles, as it should be realistically. I do understand however that you've already made it a point to say you dont care about the realism aspect, so feel free to do as you choose. Just stating my piece. Im done now, lol.