You can run the NLM on any compatible OS, including Windows XP Professional. The only limitation on a workstation vs. a server would be the concurrent network connection limit. Windows XP has a limitation of 10 network connections, so if you have many licenses then it may not be a good solution for you as there could be denial based on connection limitations.
As for your Subscription and non-subscription seats, you may find it difficult to acquire the non-subscription licenses if they have already been upgraded or cross-graded. However, if they are still current and valid, you should be able to get a license file for them. In the event that you generate your licenses all on the same day, the NLM will combine them properly. However, if you are not utilizing the backwards compatibility of the licenses (running 2009 licenses via your 2010 license file) then you can get a simplified license file that does not include this option. In that case, it would be very easy to run 10 subscriptions licenses and 5 non-subscription licenses for a total of 15 total current licenses, for example. It's confusing, but it does work. I have had to explain this to Autodesk Activations before as I do not believe they are aware that the licenses work in this fashion.
If you provide more details, I might be able to lead you in the direction that you desire.