Vidimo se u šest, zar ne?

Se is a reflexive/clitic pronoun. With vidjeti, it turns the idea into seeing each other:

• Vidimo se = We’ll see each other / See you
  Without se, Vidimo would just mean We see (something/someone) and it would feel incomplete here.

It’s present tense in form, but Croatian commonly uses the present for planned/expected near-future events, especially in everyday speech:

• Vidimo se u šest. = We’ll see each other at six.
  This is similar to English I’m meeting him at six (present continuous for future).

With time expressions, u + accusative is used to mean at (a specific time):

• u šest = at six
  Here, šest behaves like an accusative time expression (you won’t see a visible ending change on the numeral).

Yes, u šest (sati) is common. Sati is often omitted in casual speech because the meaning is clear:

• Vidimo se u šest. = See you at six.
  If you want to be explicit, you can say u šest sati.

Common options include:

• u šest i trideset / u 6:30 (often spoken like u šest i trideset)
• u šest i petnaest / u 6:15
  In everyday Croatian, people also use:
• u pola sedam = at 6:30 (literally half to seven)
• u četvrt do sedam = 6:45 (regional/colloquial)
  But your sentence structure stays the same: Vidimo se u ...

Zar ne? is a very common tag question meaning ..., right? / ..., isn’t that so?
It signals you expect agreement or confirmation:

• Vidimo se u šest, zar ne? = We’re meeting at six, right?

Zar ne? is neutral and widely used. Alternatives include:

• jel’ (je li) tako? = is that so? / right?
• je l’ da? = right? (very common in speech)
• OK? is also used colloquially.
  For a more formal confirmation you might use a full question: Je li to u šest? (Is it at six?)

Because zar ne is a tag appended to the main clause, it’s typically set off with a comma in writing:

• Vidimo se u šest, zar ne?
  In casual texting, people may skip punctuation, but in standard writing the comma is expected.

Yes. Croatian word order is flexible and changes emphasis:

• Vidimo se u šest. = neutral, most common
• U šest se vidimo. = emphasizes the time (At six, we’ll see each other)
• U šest vidimo se. is possible but less natural because clitics like se usually come early (second position tendency).

Typical replies:

• Da. / Da, naravno. = Yes / Yes, of course.
• Tako je. = That’s right.
• Ne. followed by a correction: Ne, u sedam. = No, at seven.

A slightly more formal/explicit option is:

• Vidjet ćemo se u šest. = We will see each other at six.
  But Vidimo se u šest is still perfectly polite and very common in both informal and many neutral situations.