0 offset is best for trailer wheels. It centers the load on the tread. Not saying you can't run any offset, but the trailer will tow best with no offset.
Most 6 lug will be 6 on 5 1/2, pretty much 99.999% of the time. In a 5 lugs, you're about 95% 5 on 4 1/2. It could be 5 on 4.75, 5, or 5 1/2, too. Here's a page on how you measure bolt patterns: