important thing: make sure they are definitely the same sex. do not take anybody's word for it.
quarantine: you would normally quarantine a new chinchilla for at least a month (i didn't do that with fluffy, tsk tsk) but if he'd been on his own for say, two months and there have been no other chinchillas anywhere near and he's healthy, i'd probably count that as quarantine.
now, pen. i would not recommend it. this is because i have found that when introducing chinchillas, they tend to freak out if they are in a space where they feel there's not escape from. i did think that if i introduce them in a pen, it'd be easier for me to interrupt any shenanigans. unfortunately, the dominant one went straight for the other one.. this completely changed once we abandoned the pen idea.
people have different opinions on introductions but i would say that the first meeting - or even the first few - do not show if they're compatible or not. chinchi and fluffy took over two months to introduce and until about the last week, we thought the two would never get on. and then they decided they absolutely loved each other - did everything together!
mae and spock are still being introduced - she's a rescue and has had a very hard life and a very small frame compared to spock. i would hold him next to her cage before he was neutered and they would groom faces like mad. we thought that was absolutely perfect and that it would be very easy to introduce them. however, after his surgery, they would groom faces but then he'd start mounting her and she him! we had to go really slow because she only has three legs etc but we have now got to the stage where they can be in a cage together for hours and cuddle up. still, i'm wary to just let them together while i'm not there so they still live separately.
so i'd say it's not indicative.
i would say if you do let them play together then there's not much point in doing quarantine in a different room. but i'd definitely have a vet check. and i cannot stress enough, make sure they are the same gender. you will need another cage until you're sure they can safely live together. an older and younger chin is actually a good combination but the fact that the older one is a rescue may make things a little harder, especially as his history seems a bit sad.
the car ride method, i would not do simply because i would be worried it would stress them out too much. but i know somebody on here does it and it works for them. it's a quick intro method rather than side by side cage - so you should be able to have these chins together when you get home. like i say i've never done this, but i'm not sure it's suitable for a rescue that was severely stressed not so long ago. someone will be able to help with this!
what i would do:
1/ make sure the chins are the same sex (i know i keep banging on about this but i had lost chinchi after we had to have him neutered after fluffy turned out to be a girl - she was sexed as a boy by two breeders)
2/ get the chin. have him checked out by a vet, have a separate cage set up in a separate room and do the quarantine
3/ move the cage in the same room as mei mei. start introductions. it may go quick or slow but hopefully by the end of it they'll be two happy monkeys