Asked my GM about this because I'm currently looking at picking a new spell and I really need to know what the RAW is for this, as well as RAI as my GM is willing to at least listen to the argument.
Dragon Form (SU) states:
At 7th level, a dragon disciple can assume the form of a dragon. This ability works like form of the dragon I. At 10th level, this ability functions as form of the dragon II and the dragon disciple can use this ability twice per day. His caster level for this effect is equal to his effective sorcerer levels for his draconic bloodline. Whenever he casts form of the dragon, he must assume the form of a dragon of the same type as his bloodline.
So the crux of the question is if I have in my spell list Form of the Dragon, am I really required to only take the form of my bloodline dragon. Or does this only apply to the use of the supernatural ability.
One could, in fact, read it that the SU ability isn't restricted since you're not casting the spell but using the ability, and the restriction applies only when you *cast* the spell.
Alternatively, you could claim that it applies to both casting the spell (and potentially it's variants) and the supernatural ability. The reason that I'm asking is that I was thinking of taking the Form of Alien Dragon. I so want to change into some of those dragons. But depending on how you read this, it may be completely useless because I can't change into anything but a Bronze dragon.
I believe that the intent is that the restriction only applies to the SU ability, but that isn't really what it says.
I don't know anything about alien dragons, but Form of the Dragon 1-3, all specifically say chromatic or metallic dragons as far as the spells are concerned. And the spells contain no such clause of a specific chromatic or metallic dragon.
It literally makes no sense for a supernatural ability to be capable of transforming into something other than its supernatural bloodline type of dragon. And, the supernatural ability states it must match your bloodline... Whereas the spells do not. The spells are available to people without draconic bloodlines, so the supernatural ability is restricted beyond the spells.
My bad, I thought it was SU not SP. Not that that changes things all that much. But I guess I don't see how having a SP ability interferes with casting the spell using personal spell slots. Note that while I agree the FoAD != FoD, but Dragon Disciple was written before the existence of FoAD, and given that there are now bloddlines for the Alien and Exotic dragons, it would clearly should extend to those for Dragon Form. If you argue that FoAD != FOD, then a Sorcerer(Time Dragon bloodline)/DD couldn't use Dragon Form since clearly FoD doesn't work. But the question remains, should the SP ability really force the sorcerer/DD who *casts* FoD/FoAD/FoED to only his/her bloodline dragon, or is that restriction only applicable to Dragon Form.
If you're casting it, be whatever dragon you want, I'm pretty sure. If it's the Dragon Disciple ability, it is restricted to your bloodline, to my knowledge.