Mixing some organic matter(compost, leaf molds, etc.) in the planting hole is the best fertilizer for Rose of Sharon Plants. I wouldn't fertlizer anymore than that as they are quick growing shrubs. The optimum pH range is 6.0 - 7.0 for these shrubs. The addition of organic matter will also increase drainage in clay soils, and will allow sharp draining soils to hold more water. I would space them 4' or more apart; depends on if you like the hedge look or would like a nice break between shrubs.
Have fun planting them.

Heh. My mother gave me a huge bag of Rose of Sharon shoots one year. I literally poked them into our tough clay soil, and now, nearly 3 years later, every single one of those twigs is a large (over 6 feet) of shrub. I'll say that you really don't have to do much to help RoS grow. Have lots of fun! They're truly gorgeous!