You can forget both the C-130 and the Transall, on economic grounds as well as certification, etc., etc.
Brymon Airways operated Dash 7's out of the 2199'/677 Meter runway of Baltasound Airport, on the island of Unst. These operations persisted for about twenty years, to full Public Transport rules, hauling oil company employees who transferred to/from helicopters for the short lift to/from the North Sea platforms. Carrying hundreds of thousands of passengers during this period, to the best of my knowledge the Dash 7 never put a wheel off the UNT runway.
Although St. Mary's is slightly shorter than UNT, the commercial payload of the Dash 7 would certainly be more than the Twin Otter, so it would probably boil down to the sector length desired. A look into the Approved Dash 7 AFM would provide the answer for you.