I need a better understanding of what I should do when setting up my new outdoor circuit. I've got a circuit that runs about 150 feet, so I'm installing 10g wire to allow for the voltage drop. I read in the Home Depot Electrical 123 book that I can have at most a 15amp circuit with 10g wire on such a long run. So I purchased the wire and have run it already. Now I have a few questions about the type of breaker and outlets/switches I can and should use.
1.

If I have a 20amp breaker in the panel (not currently used), can I use it and put 15amp outlets and a light switch on that circuit or maybe just change everything to 15amp?
2.

Since it's outside, I will be using GFCI outlets, should I use 15amp or 20amp GFCI outlets?
3.

Does it matter if I have just one or if I use all GFI outlets?
Thanks in advance!!!
Gabriel