I think I'd much rather Nick Brawl risk the beating-up-infants moral concern than use the All Growed Up versions of the Rugrats - it's like having "slow Sonic" in Smash, you're stripping the characters of their main defining trait that most if not all their moves would likely revolve around.
When I hear the name "Tommy Pickles", I think about the screwdriver, the ball, squirting his milk bottle, his indefatigable imagination - all those are things I'd want in a Nick Brawl moveset and AGU Tommy likely wouldn't provide. I guess he could have imagination-themed moves on virtue of being a filmmaker?? Doesn't really click for me.
There's tons of really good solutions to the dialema don't involve AGU too - my personal choice is Stu running around with Tommy in a baby carrier, with Stu being the one to take the blows and Tommy doing all the work, but I've also seen the Reptar Wagon thrown around which could be really fun (especially if it transformed into different types of vehicle through the power of imagination and all the babies were in it at the same time)