As most cog threadings are similar you can use lots of different cogs without caring about threading. However, once a hub has gotten used to a particular type of cog you shouldn't mix ISO, British etc.

See this thread for advice:
http://www.bikeforums.net/showthread.php?t=194545

If your cog won't spin on by hand, don't force it. Clean everything and try again. Cogs should spin on quite easily by hand.

Nice score btw