How tight should the cones be? How many cones do courses usually have?
How tight depends on how steep the slope is and what type of course you want to set. If you made the 7-footers first try, and soon made 6-footers easily, either
a) you've got a nicely set up board with loose trucks and a short wheelbase
b) your hill is not overly steep (as you say, that's a good thing for beginners)
or
c) you're a prodigy!
Regardless of which of those is true: whatever you're doing, keep doing it.
I did slalom when I was a kid on what I'm guessing was a similar street--just a very mellow slope--and with similar "cones"--I used old rollerskate wheels, and even rocks, as cones. When a car came down the street, my friend and I would wave them around the side of the "cones."
Like you, I was having a good time just riding and having fun. I felt like I was into something that most people had no idea about--my own little secret. And I was pretty stoked on my ability to rip through the course.
What I didn't know, however, was that most slalom courses aren't straight. They have what are called "offset" cones, which means that they are not set in a straight line, but are "offset" from it.
When I started out, I always set my cones in a straight line. I never thought of doing anything else. And when I got back into slalom in 2002, that was the first, and only, kind of course I saw. "Oh, yeah, I remember this," I said to myself. So when I went to my first race, I did great on the straight cones, but I was really sketchy on the offsets, because I'd never ridden them.
So ANYWAY...to get back to your questions:
Most courses have anywhere from 30 to 50 cones, although they can go as high as 100. And as Chris said, the spacing usually ranges from a minimum of 5 feet to a maximum of whatever the hill will allow. (Steeper hills with higher speed require a little or a lot more distance between cones.)
So my answer to your questions is: First, above all, keep having fun. Second, if you start finding that straight courses are too easy or boring, try tightening up the spacing even more, or start adding in offset cones at the same spacing and notice how you have to adjust to them. If 6-footers are easy, try 6-footers that are offset by 6 inches or more. There's really no right or wrong, just easier and more difficult.