Should we have a swarm-robotics tag?
I mean, it largely differs from robotics (especially thanks to its unique challenges), then I realized we don't even have a robotics tag.
A tag should be widely enough applicable that we can envision a reasonable number of on-topic questions that would benefit from it for categorization purposes.
The best way to demonstrate this is to point to several questions that you feel would benefit from the tag. In this case, one is none.
Also, the tag must not be a duplicate of an existing tag. (Specializations is another matter, like we have mythical-creatures but also centaur or unicorns, for example. I'm not always happy about those, but at least one is generally pretty clearly a subset of the other.) If a tag is hard to find, we should consider renaming or synonymizing it, rather than creating another tag.
While it is true that invoking extremely small machines (nanotechnology) poses somewhat unique challenges, and that what they use for weapons may therefore differ from that used by larger entities for similar purposes, I'm not sure you have sufficiently demonstrated:
For the time being, I think that the case is not compelling for a swarm-robotics tag.