SSEA is to fly simple, single engined aircraft, SLMG is to fly self-abuse motorised gliders, microlight is to fly microlights. If you want to fly an SSEA, that's the rating you train and apply for; if you want to fly SLMGs, that's what you go for; if microlights are your thing then, it's the microlight rating for you. If you want to fly more than one class of aircraft then you do the appropriate training and apply for more than one rating. No more complicated than that.