iPad Confused-Installing ios8 on iPad Air

I have 2 burning questions, would be great if someone can help?

1. Update is 1.3GB...yet it says I must have 6.9 GB free....what's up with that?

2. Does installing iOS8 right now render any of the usual apps unusable till upgrades come out...I dont want t5o instal iOS8 and then nothing works well.

Thanks guys