There might not be such thing as a DH6 type rating, but there is obviously a PPC you can get on it,which is useful.
If you had a check out on the Twin Otter it is valuable to all the employers regardless of total time, at least that was what it was like when I started out. I only assume it's even better now with the way the industry is.
I did the DH6 Flight Safety Course through one of my employers and it was alright. Had a nasty 2am slot with an instructor who drank before sim and never flew the machine (it's always nice when they have flown the aircraft they teach but not imperative) The course does not provide you a PPC because it's a relic of a sim, level C I believe with no side visuals.
Anyway, it's one of the most amazing planes I ever flew, I hope you get to too.
Puff