As for questions 1 and 2 (experience based on Fokker 50, CAT B):
I wouldn't really go any faster that 10 kts (dry runway) taking a 90 degree turn off the runway, it gets quite uncomfortable quickly (don't want to slam pax into the walls of the cabin). On determining the exist speed, for me, far and foremost the runway and taxiway conditions are most important. Take care when on contaminated runways and taxiways and extra care when in cold weather conditions. Besides that taxiway width, markings and visibility and aircraft CG position.
If the RET is designed for a 30 kt turnoff I'd do it at 30 kts depending, again, on conditions, taking care on heavily contaminated runways/taxiways.
Keep in mind that the Fokker 50 is an aircraft which will very easily slide on its nosewheel, like an "understeer" condition in a car.