Firstly, is it safe to mix green threads such as eventlet or gevent with python native threads from the standard library, i.e. Lib/threading.py in the same python process?
Secondly, if it is safe, is it a bad idea?
With gevent < 1.0 it is most likely a bad idea, as libevent isn't thread safe. You might be okay if you make sure that the OS threads never interact with libevent… But that could be tricky, and bugs cause by libevent's lack of thread safety don't sound like any fun to debug.
However, gevent >= 1.0 uses libev, which is entirely thread safe. So (as far as I can tell) there's no problem mixing green threads and OS threads.