I find this varies from library to library so you have to as them. Unfortunately, the terms “alt mix” and “stem” get interchanged and used incorrectly by lots of people including libraries as well.
In theory, an alt mix is a track that is stand-alone, to be marketed as an entity to itself, not a part to be re-mixed with other partials. But a stem is part of a track intended to allow the editor to do a new mix from some or all the stems.
Most of the reality TV cue libraries I work with want alt mixes as stand alone. No blank spaces up front, a proper ring out, mixed to sound good alone, etc. There is no concern about all the alt mixes lining up to be mixed together. However, some libraries really want them more as “stems”, allowing editors to make new mixes. **Usually**, that only happens with high end placements such as feature films but there are exceptions.
The bottom line is ask the library for their specific requirements. The only standard is there is no standard. 😉
HTH 😀