Can I just do a few dreads and leave the rest of my hair undreaded?
Totally. It's like having a few "pet dreads". It's also a good way to get a feel for dreads if you're thinking you might dreadlock all your hair at some point.
It will be important to keep your straight hair out of your dreads and the other way around. The curlier your hair is the more attention this will require. Usually it's not a big deal at all. You'll just find your dreads after each shower and pull them way from any hair that's not supposed to be in the dreadlocks. In the beginning you'll have a band on the roots and on the tips which will help the un-dreaded hair and dread stay apart. After the single dreads mature just a bit there will less and less tendency for them to intermingle.
You'll also want to maintain the hair that is dreaded the right way while not disturbing the parts and areas that are undreaded. The dread soap can be used on dreaded and undreaded hair so that makes washing them easy. You'll want to take a second and squeeze the extra water from the dreadlocks because they dry slower. I'd avoid using conditioner since it will slow or prevent the dreads from locking and it has a tendency to build up inside dreadlocks.
Most people wash regular hair every day and dreads are usually washed every 2 days or every 3 days. You could try to keep the dreads dry during some of the washings - a plastic bag with a rubberband would do it - but that would be a hassel and it's not really necessary. If rubberbands are used and the rest of the maintenance instructions are followed, washing the dreads more often won't cause them to fall out. Early on you'll have more loose hair to pull in and you'll need to pull it in more often than you otherwise would to keep things tidy. If you had an entire head of dreads and tried washing them every day, the extra loose hair would be out of control and it would take too much time to manage each day, but with only a few dreads it won't take that long, and after the first 10 washings they should have less and less loose hair.
Accelerator and Lock-Pepper are a bit tricker. They will make your straight hair feel coarse and want to knot but they can still be used as long as you take care to swing the dreads off to the side before spraying/peppering them. The wax is much the same way. Usually when people are only doing a few dreads speed is less of a concern so you can use very little or even no wax. This is especially true if your hair is quite curly and fine, and also if you're making larger sections.
Some people forget to decorate their dreads when they only have a few. This is a reminder to snag a couple beads. Other than that have fun with your pet dreads! =]
Partial Dreading: Only a single, a couple or just a few dreadlocks, or just one dread