this work as polymorph spells, you don't need the form you take to have the specific fly speed and level, only to have fly speed at all.
-it work like the beast shape and other spells. IF the form you take has ANY of the abilities the spell say it give you change what you get to what the spell give. be it energy resistance, movement rate fast healing dr etc. it is done so to the balance the given powers with the spell (or wild shape ability) level.
taking a shape of a fire elemental(with elemental form spells), for example, would give fire resistance at lower spell level which would be better at higher until fire immunity is granted. even though the original small elemental that you copy with the lowest level HAS fire immunity.
If you beast shape iv into a strix, you get fly 60 (average), not fly 120 (good).
I don't know of any humanoid creatures with fly (good) or better that are valid for alter self.