Can I set up a tamagotchi on at night?

So I got a new tamagotchi ON, and I won’t be home until 10-11, if I set it up then will it instantly go to sleep?

Nope, it'll go through the 1 hour baby stage with the meters dropping like crazy, then it'll evolve into a toddler, and then it'll go to sleep. That's how it is for most tamas except, maybe, the 4U. I also did start up my Pastel Meets (a Japanese On) at 11-ish, so I can confirm it still does this. The baby stage on practically all tamas is basically a short, intensive period which is why I would recommend only starting up a tama when you have a bit of free time. Once you've run enough tamas, the baby stage just seems to fly by.

In the case of the On/Meets, there is a way to make the baby stage easier. What toddler it evolves into is determined by the happiness level and the On/Meets has a really long bar. So a trick for it is to max out the bar and then just focus on keeping it full and cleaning up after it, as the bar wont empty till almost the end of the baby stage. If you fill it up when it gets low after its nap at the 30 minute mark, it will definitely not empty and you can leave it alone till it evolves - making sure to feed it and clean up after it, of course.

Care mistakes in the baby stage don't count, so don't worry about it. You might also want to take advantage of filling up the happiness with the teddy bear as this is the only stage where happiness goes up very quickly.