Up to your GM, but I'd interpret the RAW text as you cannot start with it. Period.
If it was simply Restricted Gear Quality accessible, it would simply list an Availability of over 12R or F - instead, this is the
only thing they come out and say "You shall not start with Adapsin."
Of course, your GM can let you start with a stealth fighter, if they want. *shrug*
Sort of off topic, but as long as I'm playing hypothetical GM - the SWAT thing...
[DISCLAIMER: Putting on my GM hat - reader discretion is advised. Nothing to see here. Please move along.

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If I were GMing, I'd probably let players get whatever Restricted Gear they want, along with a warning that there's consequences for playing with Big Boy Toys. SWAT teams may only have so many members (probably 12 or more) but at least two are usually snipers.

If you get them, too, well...congrats, you've just taken out an entire SWAT team. The gloves are definitely coming off, and instead of Knight Errant SWAT you're dealing with the Feds or the Military...not really sure which would be worse, but either way you've got an appointment with something that a single grenade launcher can't deal with. Oh, and Knight Errant is out for blood, and likely has Prime Runners on the way to do a little "damage control".
What's that you say? No witnesses? Come now...
someone knows where you got that shiny new MGL-12. Santa certainly didn't bring it to you, and even if he did...well, those Federal boys aren't too shy to use Compel Truth and maybe call in an AI to track that bad boy down.
-Jn-
Ifriti Sophist