Don't know a lot about fins on Shrooms, I have the 8'11 and running the factory fins, just cleaning out my garage and found a new pack of large Luke Egan fins, might sound silly but I can use two of a thruster set and change centre or sides of a quad ?
You can put in anything that fits... might not be perfect, but it will change the way the board surfs.
I doubt that you would like really big fins in the back positions.
However, I did like these 3.5" fins in the back boxes.
Really made the board slide'y and loose!
I've got a 7'6 Shroom with stock fins in and they're great TBH, I have tried it with the rears removed and Twinzered it for slide/skate fun but it's happiest with the rears in. I have owned an 8'3 Shroom (sold now) that I tried without the rears in and that was cumbersome because the tail is quite wide, so surfed that with the fins in too.
The 7'6 I have tried with some FCS 7's in out of my Flow, but it was very skittish and didn't really "drive" through the mush.
I've not tried smaller fins in the rears like STC has, I think that might work and worth experimenting one day, but my overall feeling is the swept back fins Bert designed for the Shroom work perfectly well. It's the only board in my Sunova quiver that I haven't swapped out the fins for some other FCS variant.
No harm in trying IMO though, your 8'11 might just like a swap out being a bigger board and all that.
Try it, let us know how you get on Yeah?