1. Dealing with fewer people is always easier. There's always a hassle of signing contracts/making payments etc.. As well the quality will be more consistent
2. Payments vary.. but if it's an artist you never worked with it's not uncommon to use a deposit/final payment structure. Like.. x% up front and rest on delivery. This varies with the size/length of project and whether its open ended or finite.
3. If you think you'll have more than one artist then I'd recommend choosing a main one to act as the designer/art director and having him set the style more concretely before bringing others on.
4. There are some floating around the net if you look.. Don't have specific links.
- avoid shared royalty deals if you're small time.. its too much accounting work. If you MUST include a % of royalties as part of the deal to get the guy you want attempt to limit it to certain milestone payments.
- make sure the artists you hire have a good track record. Get references if you can. It's not uncommon to run into problems like.. artists stealing work from copyrighted works and presenting it as their own etc...