Hi all, does anyone know the answer to the question.. I just cannot seem to find the answer anywhere and if there not interchangeable we have spent a fortune collecting characters that cannot be used..
No, Giants won't work. Swapforce, Lightcore, Series 2, and Series 3 will work. I know very little about this game (despite selling it every fucking day...) but the official Skylanders website (linked) has a breakdown for what games support what character series.
Swap Force is the same as Giants was to Spyro's Adventure. All of the characters from the previous game are forward compatible to the new game. All Spyro's Adventure characters can be used in both Giants and Swap Force and all Giants characters can be used in Swap Force. Any new characters introduced in a newer game are not backward compatible with the old game because they were not written into the code of the old game. If you pick up a Series 2 or 3 of a character or a special edition like Legendary or Light Core they are backward compatible to the old game even if the box says they are not but will play in the old game like the original character.
In Swap Force all the previous magic items work as well but portals don't take you to those adventures any longer, they play as new weapons in the new game instead.
The old level packs and portals serve a purpose, they don't play as the same adventure like the Spyro's Adventure ones did in Giants but instead are weapons. Dragon's Peak drops fire on the minions etc...