Is it legal and "workable" to use a Mac Pro as a thin server for Mac OS? I have a software company and my developers each have their own preferences for an OS (Mac, Linux based or Windows).

For windows I know you can use remote desktop and stuff. I know it is possible in Mac as well. But is it viable to let the employees that prefer Mac work from 1 thin server on a powerfull Mac Pro? If yes: How many should I expect to be able to work simultaneously? Does TightVNC as client give good results or is it tedious (slow / bad graphics)?

I like to dream that everyone can use all the computers available on every workplace, because they work remote and don't have to use a specific computer each time to access their programs and files.

Yes I can create network shares, but sometime someone who doesn't prefere to work on Mac still has to develop on one because of XCode for example and we could buy Macs for these situations it's not ideal just for that instance.
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.