You’ll only need one device type handler for each model. Then you assign each individual device of that model to that DTH. So if there’s a DTH that works with nest and you have 4 nest thermostats, you still only have one copy of that DTH in your account. The DTH is essentially a translator between the SmartThings hub and the device. So it’s like loading a French translator into your word processing program. You only need one French translator even if you write seven different documents.
That said… With regard to the specific devices you mentioned, nest and blink, the best that can be said is that they work today. Whether they will work tomorrow, there’s no guarantee. A DTH for a proprietary API, such as those have, is a very different situation than a DTH for a third-party certified device, like a Zwave switch.
The blink integration, for example, stopped working briefly earlier this year, and then was brought back by customer demand (to blink). There used to be a MyQ integration, and then Chamberlain shut that down from their side and it never came back. Netatmo used to have a semi official integration, but that has now gone away. I don’t recall whether the unofficial integration still works with that one or not. IHome has an official integration that stopped working for some devices last month, and SmartThings is working on trying to get it going again.
So the short answer is that at present, what you described will work through unofficial integrations. It occasionally happens that an integration will stop working or will drop some features. It’s not common, but it can happen.