working with white paint.

If it's less than three (3) hours, that should be fine; otherwise, you'll need to wait at least 48 hours to re-coat, or you'll probably get the dreaded "orange peel" effect.

If you don't get runs, it is okay.  You are probably putting on thin coats.

I don't wait for the paint to dry between coats. I spray, wait about 10 minutes and then spray again. Is this good practice?

I have my favpirite white paint and usually I don't have problems with it.

Tamiya's bottled primers are not expensive at all. A bottle costs $3.80 and is good enough for 15 kits.

Whites and bright yellows are the most difficult for me to work with. When spraying you tend to over do it because it doesn't look like you are getting any coverage. You spray too much and then it begins to run. As mentioned above apply it in several light coats. It certainly doesn't help that these colors, especially enamels, take longer to dry. It may take several days to get a solid coat because of having to wait for the paint to dry between coats.

If you are looking for a really smooth glossy finish I recommend using one of the finer white primers like Tamiya. This stuff is expensive so I save it for when I want that special glossy shine.

There are flat white primers available.  Krylon makes a good one in spray cans.  I almost always use a white primer for both white and yellow paint.  I find primer even more critical for gloss paints than for flats.

I get best results with white (using any brand of paint) by doing several layers. First up a rough layer of flat white as a primer to give all the next layers something to bight on to. Then I will do another layer or two or three of satin, semi gloss or gloss white, depending upon how intense I want the white to be.