I have been breeding for 18 years now. Here is what I use.
3 females and 1 male per cage. Male is never removed.
I feed Mazuri rodent ad lib. 6F I think is the product number
20 gallon long tanks with toppers
http://www.martinscages.com/products/cages/toppers/
aspen bedding and clean as needed
I put card board boxes and PVC pipes in the cages for enrichment and nests.
As far as the nipping. They are just checking you out.