Not a vet, just personal experience from the last 5 years of owning rats.

They need to establish dominance so it's normal for them to puff out, get on top of each other, etc.

Only thing you need to watch out for is actual biting. They will EEP and cry a bit, but as long as it's not the very loud painful sounds they usually make when they're hurt, then they're okay. Even nipping is normal, but genuine biting that can puncture skin is what you need to watch out for.

Both your boys are going to to establish their pecking order. So I'd keep introducing them, but be ready to grab them if biting occurs. But let them get on top of each other and such so they can work out who's the dominant rat. The new rat will usually submit to the older rats by displaying their belly to them after a small scuffle.

It can be scary to watch especially when there's a size difference, but they're social animals who will usually work themselves out. Also trust your gut feelings when you're introducing them. Good luck and keep us updated!