Commercial Plugins and Solo Licensing?
Technically, that's up to each individual plugin developer. But realistically, the vast majority of commercial plugins are only valid for a single site.

If it's a paid plugin, you'll probably need to pay for it on each site independently.


  "I may power more than one site for free under the terms of the Solo license."


It's important to realize that you're actually working with multiple licenses in this case... they just all happen to be Solo (aka free). Each site has its own license, you are not sharing a Solo license across multiple sites.