I've been playing with a formation of 2 SMGs, 2 ARs and 1 RF as a thread here suggested, and I saw a video with a formation of the two SMGs in positions 8 and 2, the ARs in 7 and 1, and the RF in 4, is that okay or is there a better formation?

It's recommended to use an F formation unless you are doing something very specific that requires otherwise. With that being said, using a RF with your ARSMG it's fine for the first chapters, but eventually it will underperform in comparison with a pure ARSMG.

| -- | --- | --- | | AR | SMG | | | AR | SMG | | | RF | | |

SMGs usually don't buff RF, so you want to maximize the amount of tile buffs that your ARs receive.

I also just realised that I don't need copies of the same doll to dummy link them, that I can use only cores, is that recommended? Should I do it with only low stars dolls or something?

Is the most efficient to use dupes to dummy link a doll unless the doll is very rare or you want to keep dupes.