So I was wondering, are you able to get the "real" homes for your 3th, 4th and 5th villager if you adopt them from somebody else? Or will these villagers always have the starter homes no matter what?
If you mean the 3 plots that Tom Nook has you set up at the beginning of the game, I'm pretty sure you can't adopt from someone else. I remember having someone try to get Beau from me a few weeks after the game dropped and it wouldn't let them. Afaik you only have the option of visiting mystery islands or else the plots will get sold to randoms by Tom Nook after you place all the necessary furniture and speak to him.
So in short, yes they'll always have the default starter homes.
It is annoying, especially if you're lucky enough to find Audie/Sherb/Megan who can't be scanned in via campsite. The only way to get rid of the starter house is to find someone willing to adopt the villager from you and then kick them out so you can adopt them again.
If you kick them off your island and move them back in, they should have their original home. There's a thread
New Neighbor Network where people can hold your villagers so you can move them back in. When they move back in, they will have their default homes