No because they came out after giants
Yes, the Spyro's Adventure Portal will work on Giants. So you can buy the Portal Owners Pack.
Yes
Of course.
Giants is the second game of Skylanders. Your current Skylanders can still work for Giants. There will be bigger figure and characters you can buy and new story.
no you cant
yes they do
Yes, you can.
No or maybe hasn't been confirmed yet. But if they do, make sure that you look out for any Skylanders Giants news
no, you cannot use skylanders from skylanders sypro's adventure
yes
yes there is going to be a skylanders 3, it is confirmed in the credits of skylanders giants