Is it legal and "workable" to use a Mac Pro as a thin server for Mac OS?
Can you use VNC (or ARD or some other remote desktop software) to remote into a server to do development work?

Short answer is "yes."

However, it is a royal PITA. I use VNC to administer my OS X machines on my gigabit LAN and I can tell you that the lag is infuriating.  Trying to do development work, I can only imagine it being a nightmare.

What worked for my developers was that we had several MBPs that would float around the department depending on the project.  For "quick" tasks VNC worked, but when they needed to be heads down on a project, they would check out a MBP and use it for the duration of their task.

The main thing to keep in mind is that people like what they like - Mac/Linux/Win.  The "hotelling" concept is a nice goal, but not feasible for devs who spend their lives in front of a monitor.  Ensuring work is accessible anywhere is the reason we have a cloud and GitHub.  These are areas you should investigate thoroughly before you spend time trying to re-architect your infrastructure.