There are no megas. Super to Super of the same morph would produce all supers. So super pastel to super pastel would produce 100% super pastel.
Super to Super of different morphs will give you 100% of whatever the two genes are. So Super pastel to Black eye lucy (Super fire) would give you 100% fireflies (Pastel Fire).
Genes come in pairs, so supers mean the animal has both copies of the morph gene.
I hope this helps.