"Culture" is just shorthand for talking about the common sets of behaviours between groups of people.  You can make it as small or as large as you like.
Humanity at the broadest scale has a "culture" that can broadly be defined by examples like "pleasure is preferable to suffering", "we take care of our children", and so on.  But you can also go the other way and take it down to the individual level.  No two individuals have completely identical beliefs (or even consistent with themselves sometimes!).  So ALL interactions between people could involve some level of "multiculturalism", purely depending on where you choose to draw the lines.
Even at the less extreme ends of the scale, is it "multiculturalism" when a West Berliner and an East Berliner come together?  How about a Berliner and a Münchner?  What about a Münchner and a Parisian?  Or a Parisian and a New Yorker?  Or a New Yorker and a Hong Konger?  Or a Hong Konger and a Mbabaner? etc etc.
The entire concept of it being problematic for cultures to mix is fundamentally an anti-human position, and is generally just an excuse for racism or other kinds of xenophobia.