Ok back again with insomnia. Will make another attempt at those questions.
1) Soil for semps is ideal when the soil provides good drainage. Various amendments that can improve drainage is small rock or grit (volcanic, perlite, large grain sand). Top dressing is good when it can keep the foliage dry on top of soil to prevent rot. Semps do not like their feet kept wet so a good thorough watering and then a period of drying in between is good.
2) Different semps have different growth rates. Differences in the conditions for growing will affect rate of growth and spread. Size of plant and growing habit of plant will affect appearance and growth of spread. Small semps aren't going to appear to spread as far as bigger ones. Some semp types mound in growth so spread is smaller appearing and others spread out their offsets (chicks) on long stolons and so enable to the chicks to establish themselves further away from mother hen.
3) I guess putting semps anywhere in close proximity and in uncontrolled environment would contribute to hybridization.
