Where in the world do people continue to get the impression that people can morph into elves and dwarves at puberty? Goblinization is rare - still possible, but rare enough - but that's still restricted to orks and trolls. If they're an elf or dwarf, they were either born that way, or they got messed up by Halley's Comet during the SuRGE year of '62; this is the only time in SR canon that a living non-elf/dwarf transformed into an elf or dwarf.
When it comes to the 'pyromaniac elf decker with a flamethrower who hears voices insisting the world must shape up or burn, a flame spirit ally, a troll sister, and human parents', the only thing that would give me pause would be the flame spirit ally - if by ally it is meant 'ally spirit', as per the actual rules, e.g. a specific spirit that a mage designs and conjures (and pays karma for) in order to have its assistance in his magical workings. Elf decker? No problem. Pyromaniac? Go ahead. Hears voices telling him to burn things? If that's what you want. Troll sister? Sure, sure. Human parents? Fine, fine. Ally spirit?
... let's just say 'are you suuuuure???' Because the GM smiling beatifically at something that might be a messed-up request should send every player screaming for the hills.