The lore does not explicitly support any other type of dragon than what you can make in the character creator. Istarian dragons are aligned with fire, but can utilize other elements, usually as breath attacks. Dragons from other realms that are non-Istarian and are aligned with other elements are out of lore in the Istarian sense, but the fact that there exists a portal to other realms in the world, it is arguable that they could travel to Istaria.
Soul-sucking, elves or not, is supported through the Spiritist classes that bipeds can take. Many of the spells for those classes are drain-based.
Dragon-slaying gnomes would work so long as they don't mind being branded murders by the Empire and probably being hunted by dragons for the rest of their - likely few - days.
A dev recently said that the blight and the Gift do not really coexist, so technically blighted characters that are truly Gifted break lore.
Player characters are Gifted in game mechanics because they have to be, but it's really up to the player whether or not their character is Gifted.
I should hope dragons read books, what with all those libraries full of the things. :P
The lore does not directly support the existence of the planet we players live on, if that is what you're asking. There is a Realm of Earth, I believe, but that's something totally different.
Dragons are said to be fire spirits that took physical form in the Prime, but I cannot recall if that is legend or if it is actually fact.
None of these are covered in the lore as far as I know. As for ferals, anything can become feral if it's born in the wild and has no community to raise it.. a dragon can't magically learn language and that it's bad to kill people and that not everyone is out to get it after all.
As to pets, I would wager that any biped that is known to be keeping a dragon as a pet or a slave would soon find themselves beset by a number of angry dragons.
I can't recall if it's fact that you can't know you're Gifted until you die, but it's pretty heavily implied by the starting experience, at least for me.
The hatchlings/adult question is not really answered. I imagine if an adult Gifted dragon died, they would return as an adult. It's just game mechanics that only hatchlings show up on Skalkaar, imo.
I don't believe the memory question is really answered, but I can't remember for sure.