See [this post]( Mixing Unity tiers in a project? page-2#post-2720730) from Unity. If you’re a freelancer, and not an actual employee, you aren’t required to use the same tier. Quoted:
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The intention here is to ward against orgs that have just one Pro (or Plus) subscription and everyone else on Personal. If you’re someone that’s working on a project as an independent contractor for someone, we don’t want to restrict who you’re able to do business with. So long as the people you’re talking about are independent contractors (by way of payroll, taxes, etc.) then it’s fine. However, if they are actual employees of your organization then it will be in violation of our EULA.
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If you import paid assets from the Asset Store into the customer’s project, the customer must also buy a license for the asset. If it’s a per-seat license, the customer must buy a number of licenses equal to the number of employees who will open the project. Even if some of those employees won’t use the asset directly, as long as they open the project they need a license for assets in the project.
[EDIT: Sorry, poor memory! After the thread linked above, I posted in another thread that I opened a support case to Unity’s legal team and they confirmed the opposite – that the EULA is indeed correct, and if you’re freelancing for a company that requires a Pro license, you also require a Pro license.]