As a freelancer working with Unity, I might end up working with some employers that use Unity pro, and others that use Unity free.
Say I’m willing to purchase my own Unity Pro license, I’m still gonna have to work with Unity free for those employers that do not use pro.
How would one go about that?
Code or assets you develop in Unity Pro will still compile in Unity. Some features might not work, though. For example, if you implement OnAnimatorIK() (a Pro-only feature) for a Mecanim control script, it won’t get called in Unity.
For other features, say LoadLevel() vs. LoadLevelAsync(), you can check Application.HasProLicense() to determine which to use.
If you want to test your deliverables in a Unity environment, you’ll have to set up a separate machine. It could be a virtual machine. Licensing is done by machine; you can’t run Unity Pro and Unity on the same one. But you can run different versions (e.g., 3.5.7, 4.3.0, and 4.3.4) on the same machine as long as you install them in different locations.
The exception to licensing is that major versions have separate licenses. So you could run Unity Pro 3.x and Unity (free) 4.x.
The pro licence is sold per seat so I can assume that if you are using your own equipment it would be fine, but it is worth noting that their are pro features which are excluded from the Free version (like area lighting), which would fail (and possible make things look ridiculous). You are probably better working in whatever version of Unity your employer at the time is working in.