3), 4), and 5) are correct!

For 1), I actually like your answer better than the real one, but I won't post the right answer just yet, let more people try.

2) Nope

6) Thats a good answer too, but I think they meant only use 2 numbers, like 56 or 88, not something like you wrote, because 20000000000000... or 30000000000... etc are all theoretically larger than 1000000000.... Good guess though!

EDIT: 6) 99 is not the right answer, there is a certain trick that the answer uses. Actually I don't know if "trick" is the right word, but I think you get what I mean.