Perfect! - with regard to the situations in which you would use each.
BUT: 1. Either, "I'll be back soon" or "Back soon." or even, 'Be back soon", but not, 'Will be back soon." 2. Why add, "Just expect me?" Maybe you have a reason. It just seems unnecessary, since 'popping over' indicates 'so expect me'. 3. Omit 'for some time' . The whole idea about popping in, out, or over,
The way you have justed 'pop' is correct. Eight or 8 doesn't matter is this informal usage. For formal use write the word but this is an informal dialogue and most likely to be said or maybe texted.
I wouldn't use might and the just together. I might pop over............, so expect to see me. I will pop over......,expect me.
You can't say "MIGHT pop over' and then say, 'so expect me'. If I say, "I'm going to be down in London on Saturday, so I expect me", the recipient of the message would stay in with the full expectation that the person would be calling. 'might' expresses only "possibility". Hence, I wrote:
Don't say 'just expect me' or even, 'expect me'. It isn't logical.