Calling Smart Service from within Interface

Yes, smart services can be executed in any save into of any component. You can execute only one smart service.

If the use case is not so critical, I suggest to call the smart service in the button or link. Because it is a costly computation which may slow down the interface performance if used in save into of some components like text field.

Yes you can. Can you elaborate your use case a bit? It might be that there are other, or even better options.