Seconding PPs who say don’t worry too much about getting tiny babies clothes - if they are in NICU they will be just in a nappy, and if they are in SCBU hospitals do have spares - think of all the women that go into labour early for singleton pregnancies and weren’t expecting it, the hospitals have spare clothes you can borrow for just such an occasion. Just pack for you to start with.
If you want to have a couple packed just in case I strongly recommend asking around for donations - I had some preemie clothes from my DD and she only wore them for about 2 weeks, so anyone that had a tiny baby is likely to have clothes that still look nearly new.
Likewise, ask around for car seats. I know you’re not supposed to bu second hand in case it’s been in an accident etc, but if you know anyone that only has 1 kid, there’s a reasonable chance they’ll have hung onto their baby car seat for any other children they have. If you make it clear you’re just looking to borrow for the first year and would give it back sooner if they needed it, you might well know someone who would lend you one.
It’s tough financially finding out you’re having twins. I know I felt all prepared when ttc because I’d got car seat, pram etc. Then finding out I was carrying twins my first thought was “damn! I’ll need a double pram! And how will I get 3 car seats in our tiny car?”. Got to admit I’m sweating buckets over what childcare is going to cost at some point in the future too.
Sorry to hear you had tough news at your scan. I assume they’ve been ok till now? Remember, they’ll have done most of their developing, third trimester is a lot more about gaining fat than getting organs sorted, so being a bit small increases their chances of needing a bit of time in an incubator, but nothing more scary than that. Hopefully you’ll get good news at your next scan.