Based on red devil 6900xt specs, 320mm*135mm*62 mm, seems like it will fit. The merc 319 probably will not fit as it's 330mm long. The vanilla xfx, sapphire reference cards etc should fit as they do also fit in the ncase m1.
I actually have fit two of the same fans except I use a 3080 ventus 3x deshroud. There is a 1-2mm gap between the heatsink and the fans. The noctua fans come with a rubber anti vibration sleeve which I put onto both fans thus closing the gap.
Regarding the 4pin cable, I ordered 2 adaptors from...
Sorry my bad. 20 eur was for the slim version. 30 eur is for the normal sized one. I just ordered another 92mm slim fan from them as well since they're the only ones with stock.