Never been... on the bucket list. Watched some footage from the early 2000s when the Russians started flying some crazy Americans and Scandis in and they were chucking some pretty big deer hair patterns to imitate mice (yes mice...trout were crunching these animals as a major food source)... so think big, think anything that looks like a small mouse... bit of fur with a tail etc. Ive also used the Dahlberg Diver (deer hair popper for salt and freshwater bass) and suppose the could imitate frog/mouse.
Probably also worth thinking big insects... check out the local insect life, but expect big hoppers, big crane flies big sedges.
The general gist in Kola was that everything is much bigger and more predatory. Forget the chalk streams of Hampshire with 3/4 weights and super specific flies. In this place the fish don't get to be picky. It needs to eat and eat big quickly. Rivers are strong and things happen quickly.
So also think 6-8 weight rods to muscle the fish in the fight an deal with any strong winds. And go prepared for the mosquitos.
It has to be done with official guides. I have read a field report of some hikers who tried to do a bit of fly fishing during their journey and they were reported by one of the camps to the police. Police came with helicopter. End of the story.
As for the flies, I don't know. Probably a lot more fishing pressure than the early days.
I know a guy from Germany who co-organizes fishing camps in Kola. Have a look at the pics, sometimes you can see the fly. I see salmon tubes...