500 is the default minimum, but you can set it to 100.
For me it's actually the best so far. Tried hashparty, space, electromine.fr, electroneum.ovh and a few others I can't remember, and all of them had stability issues. Nanopool seems to be the most stable ETN pool. The fact that it's 65+% of the total ETN network probably speaks for itself. Not completely trouble-free though, there's been no payouts for 9+ hours now, something's stuck I guess.