I read that Kvatch rebuilt should be installed after Better cities however using BOSS loads Kvatch rebuilt.esm first, followed by Better cities.esm. It doesn't report of any mistakes and so far I didn't have a single problem with any mod since using BOSS.
Should I change the load order anyway or it will be fine like that?

EDIT:
The game won't even load with Kvatch before Better cities so I'm forced to change load order in BOSS but don't know how to do that? Can anyone help please?

If your game crashes on startup it usually means that an esp file is missing a master. Look up your load order in Wrye bash and search for red check boxes.

btw, both Kvatch rebuilt and Better cities are quite popular, so I'd guess the boss load order is fine, but I haven't installed Kvatch Rebuilt (yet) so I'm not sure.

Install order and load order are different. I don't think it matters that much which one you install first, since they don't really touch much of the same places, if at all.

What you should do is report the load order issue to the http://www.gamesas.com/topic/1404766-rel-boss-for-oblivion, so they straighten it out. Best if you quote the Readme as well your experience confirming that.

In the meantime, to fix it for yourself you need to create a BOSS 'userlist rule'. Open the BOSS GUI and use the built-in rule editor. Instructions in the 'BOSS\docs' folder 'Userlist Syntax' HTML file. You will need the 'OVERRIDE' command as both mods are already in the Masterlist.

-Dubious-