'just' had the sense of 'that's enough for me, something simple for you to do' But 'only' is strictly 'no more than this'. So just is better in this case. :)

They are usually interchangeable. "Only" excludes other things, other people. "Just" emphasizes smallness. I assume that there are regional differences and "only" is more common is some regions and "just" in other regions. At work, only Dan was qualified to work on network problems. (= Dan, no other workers) Do you want any milk and sugar for your coffee? Sugar only, please. (= sugar, no milk) My grandson is just/only three years old. (= emphasizing the young age of the grandson) Just/only a few people showed up for the public hearing. (= emphasing the small number in attendance)

In this case, both are technically correct, but it is more natural to say "Just milk please!" You could also say "Just milk please. No sugar, thanks." I have never heard people say "Only milk." This could possibly be misunderstood as meaning "I only want milk, nothing else. (no coffee even!)"

In this case no difference! I guess it would be more coloquial to say ''just'' but both are correct!

In this case, both are technically correct, but it is more natural to say "Just milk please!" You could also say "Just milk please. No sugar, thanks." I have never heard people say "Only milk." This could possibly be misunderstood as meaning "I only want milk, nothing else. (no coffee even!)"