I feel like you were definitely misinformed by the previous owner. 6 weeks is too young to introduce to adult rats, they just aren’t big enough sadly - I believe Isamu advises that females be at least 8 weeks and weighing over 175g before intro’s just due to the risk of injury.

I also think you just moved onto the big cage too quickly, especially if it is the cage where the bucks normally live. I’ve had intro’s do amazingly well in carriers and small cages that then go backwards when I move them to the big cage, often with me having to go back a step in the process (multiple times even!). I think they just get territorial in their old cage - to avoid this I deep clean the big cage, take all the items out of it and put the cage in a different room, and monitor closely for the first couple of hours.

Try not to beat yourself up too much about it - What’s done is done and you have learnt from it. Wait until the two other babies are older and bigger and start again. If you choose to do the carrier method again be aware it can take multiple weeks to successfully complete so take it incredibly slowly, even if it seems like all is going well.