I have been concentrating on creating the Chinatown part of my neighborhood and since there are some lovely lots that I really like in Shang Simla, I was thinking of using them. Then started wondering if that is wise, since those lots have different mailboxes and even the trash cans (at least I think so because both normal trash can and vacation trash can have their own recolor category).

Before I start a major job on revising those lots, could somebody tell if this is a bad idea or if it messes up something in the world, either straight away or later while saving and exporting the world? Or whether if it is a bad idea in general? Thanks.

Just recently I build a vacation lot in China, saved it to the Bin and placed it in normal neghborhood in CAW. Since then, 2 testing versions of that world were exported and tested, and there were 0 problems with it. Mailboxes got replaced the same way LN mailboxes did.
If you want to be safe rather than sorry, save all the lots you want in bin, put them in any hometown you have, and see what happen. You can also save them from normal neighborhood to to bin and from there put them in CAW (and recolor replaced mailbox and garbage can into what you want).

Oh, that's good to know. Thank you so much for the information, Anushka. Now I can continue creating the Chinatown without having to worry.