Type ratings for turbine aircraft depends on the rules of the country. In the US, for instance, no type rating is required unless it's a jet or over 12,500lb MTOW. As long as the US licence authorises multi engine flight then away you go. Insurance is a different matter though.
For training there's no great reason why a student couldn't go from a C172 to a Twin Otter. In many respects it would be easier than say, a Baron or a Seneca 3. The Twin Otter has fixed gear and is relatively slow. Generally turbines are easier to operate than many piston engines. No cowl flaps, no mixture to lean & no CHTs to baby. Most critical things are torque & ITT limits but that's no worse than monitoring MaP to avoid overboosting a fixed wastegate turbocharger or managing CHT & EGT. The big catch is that it's easy to screw a turbine very expensively during starts if you're not watching things.
Although quite do-able it seems an expensive way to learn fundamental multi engine skills that can be learnt on a much cheaper piston twin.