@Phineas
The latest licensing guide I could find is from June 2022. There may be a newer one, but I could not find it at the moment. However, usually this specific documentation page and the link that is inside of it, has the latest version of the licensing guide, to my knowledge.
I attached the actual licensing guide here for your convenience at the bottom of this post.
They still have Portals rather than Power Pages, apologize if this is confusing for you.
Here's the problem - I am unsure that Power Pages and Portals are identical. I am not sure it is just a rename or rebrand, because as you can see here: https://powerapps.microsoft.com/en-us/blog/announcing-the-preview-of-microsoft-power-pages/ it is more than a rename of the product.
Let's say for now the licensing is the same (I am not sure it is though).

Notice here above, the per user plan has access to the Portal too, so you won't have to worry about that.
If there will usually be 500 users, even if they will change each month, each season, or otherwise quite frequently, I would recommend the per-user plan, as you would have access to Power Apps Canvas App, and Power Automate, and Dataverse, Model Driven Apps, and Power Pages, and this is probably best for your multi-dimensional scenarios.
If you go the route where you pay for just Power Pages and you pay per login, it's actually per 100 logins, not per 100 users or people - this might add up really fast and actually cost more, if people will log in frequently, than if you just took a per-user plan for each contractor.
Additionally, there would be no access to the full Power Platform - and this could really be a roadblock, and end up costing you more money in both the short and the long term, and for much less functionality. This only saves you money if you are absolutely sure they only need Power Pages, and you'll need to be sure they login infrequently, otherwise it'll probably do the opposite and cost more money for less functionality. From what you described so far, I am not 100% sure that your contractors only need Power Pages, and even if that were true, I am not 100% sure this option with paying per 100 logins, won't cost you more money for less functionality if the contractors login too frequently.
Also, if Power Pages is actually not enough or not the best fit by itself, you will end up not accomplishing your objective, which is not good.
So I recommend the per-user plan.
If you want the main company to be separate from the independent contractors, this is really simple.
Just ask for there to be 2 separate tenants
One for the main company.
And one just for the contractors.
I would recommend you go to this page https://powerapps.microsoft.com/en-us/pricing/ and click one of the three buttons on the right side and follow those instructions from there to get in touch, the official team that handles this might be better able to clarify this for you.

You will want to check the volume pricing if you're interested in paying for just Portals and really only want that, and plan accordingly, and only if you are really sure it will really cost you less (I'm not sure it will, but if you're pretty sure it'll cost you less, and only if you're pretty sure the logins and the associated cost will be more infrequent than if you have a per user plan for all contractors - and that Portals is the best fit - and it might be the case, depending on your scenario - here it is) -

Here's the problem - I am unsure that Power Pages and Portals are identical. I am not sure it is just a rename or rebrand, because as you can see here: https://powerapps.microsoft.com/en-us/blog/announcing-the-preview-of-microsoft-power-pages/ it is more than a rename of the product. I do not have any info I can find on Power Pages licensing specifically. I recommend you contact the Power Apps team using the instructions above and get them to explain this to you.
Check if it helps @Phineas