I don't like the mutually exclusive arguments either. :-1:

Two general approaches come to mind:
1. Have a single, canonical form (e.g. a list of dimension indices) which can express *all* requests, and provide conversion from other forms via other method(s).
2. Extend the string-or-coord type checking to allow string-or-coord-or-number.

Out of these two I currently prefer option 2.