As it's proposed I understand it's for everyone, the reason most of the mentions are about PI teams is the degree to which they fill the rugby diaspora, but smaller African nations like Namibia and Zimbabwe would likely benefit too.
Plenty of other sides would have an opportunity to pick up players, the difference is they wouldn't be drawing in either the volume or quality that the PI teams would, seeing them move back relative to these teams. That shouldn't be an argument against it from a world rugby perspective as it's essentially raising the quality of international rugby across the board, but these countries will almost certainly see it as a reason to vote against it.
If it were to be restricted by tier, the tiers have very clear definitions set by world rugby and teams have to be specifically promoted (or theoretically demoted) - Japan was the most recent addition to tier 1 and the only team outside the six nations/rugby championship.
I'm not sure where in world rugby's site the tiers are listed, but as far as I'm aware the wikipedia listing here is up to date:
List of international rugby union teams - Wikipedia
en.wikipedia.org
Kyle Godwin