Yes, you'd all start as neonates. While the entire cast is going to be thin-bloods, the mechanics are in the core rulebook I have, so I would allow a player who wanted to play one to do so. That said, you will be suffering all the usual stigma, and I'd have to speak with the player about why they have not been hunted down.
Actually, I just caught this part, and this concerns me more than the other things. Ignore my previous post.
First of all, since you say we're following V5's mechanics, you should know it's impossible for a Kindred to be both a Neonate and a Thinblood. For the simple reason that for one to be a Neonate, he must be (at the youngest) from the 12th Generation. Anyone from Generation 12+ is a "Fledgling" by the book's nomenclature, which does overlap the Thinblooded (from Generations 14-16), but anyone from Gen. 13 or
lower is considered a full-fledged Kindred.
As you can see, it's impossible for us to be from Generations 12-13 and 14-16 simultaneously. So I'm confused as to what you want us to play. Is our blood the blood of vampires, or are we half-breeds? Are we allowed to choose Clans or are we all Thinbloods? Because there's no such thing as (for example) a "Thinblooded Ventrue." The Thin Blood by definition is so diluted and weak that it hasn't manifested the strengths and banes of any one bloodline.
Secondly (if I'm understanding correctly that you want us all to be Thinbloods—please ignore if not), choosing our Clans for us is, by all accounts, a very bad idea. By making us all the same Clan you're basically assigning us the same niche within the group, because we all have the same Discipline spread to choose from at character creation: Thinblood Alchemy, and then whatever blood just happens to be available to us within the Scene. You're also hobbling us by making sure we can never go higher than 2 dots in any Discipline except Thinblood Alchemy. Or at the very least that it will take months, maybe years, to accumulate enough EXP to raise our Blood Potency to 1,
then start buying Out-of-Clan Disciplines at the higher price.
For example, if I want my Thinblood to reach level 3 Protean so he's somewhat viable in Brawl-based combat, he will need to spend 10 EXP to raise Blood Potency to 1, then another (7+14+21) EXP to bring an Out-of-Clan Discipline to 1, 2, and 3 dots respectively. That's 42 EXP. Instead of simply letting me play a Gangrel from the beginning of the game, you want me to spend most of a year shaping a Thinblood into an inferior version of the character I could've been playing from square one.
It's also a bad idea for the simple reason that ... this is a tabletop RP, where we want to distinguish ourselves within our coterie, and be the best in the group at something, and have our talents and weaknesses complement each other in interesting ways while we progress in this story. And by telling us what Clan we
must choose at character creation I believe you're taking most of the fun out of this creative process.
I'm sure you have a strong idea of why you want us all to play Thinblood and how that group dynamic will best benefit your story. But I hope you will reconsider this choice because a tabletop RP isn't only your game; it's your players', too. And I can guarantee you that I'm not the only one who wants to play another Clan.