I haven't tried it myself, but I hear that if you bring in the new series 2 and 3 characters (i.e. Heavy Duty Sprocket or Mega Ram Spyro) they show up with the "Special" tag in Giants (like Stone Zook or GITD Cynder would.)
1. Every character work in all later games, no matter the platform. So every Series 1 figure work in every game, be in on PS3, XB360, Wii, Wii U, 3DS, PC (Skylanders Universe) or iPad (Battlegrounds, Lost Islands and Cloud Patrol).
2. New characters will not work in earlier games. Giants were introduced in the second game, so they won't work in the first one. Swap Force figures were introduced in the third game, so they won't work in the two first games. (as far as I know.)
3. Series 2 figures will work in the first game, but will be 'reset' back to Series 1. Series 3 work in every game, but will be reset back to series 1 in the first game and series 2 in the second. This is only because those games don't have the code for the updates, so you can't go past lvl 10 in the first game, and lvl 15 in the second. Think about it: The first game had lvl 10 as the level cap, so how would it be to come back with a lvl 20 character? Overkill.
4. If you do reset a character back to series 1 or 2, it's only for that game. The figure itself hasn't changed, so it will change back to series 2 or 3 if the game supports it. So if you take a series 3 figure back to the first game, it will be shown as series 1. But if you take it back to the third game afterwards, it will be shown as series 3 again. You don't 'break' the figure by going back to earlier games.
5. Light Core characters glow in the dark, and have an AoE once pr level when you summon them. And they look cool. :)
6. The Nintedo 3DS also have Spyro's Adventure, Giants and Swap Force, but they are entirely different games from the other versions, with a different story, different gameplay and more linear skilltrees. You unlock the skills automatically, and can't choose them. Even so, the figures from PS3 work on 3DS, and vice versa.
7. For collectors: Once you summon a figure, you have 'claimed' it in the game. But you don't own the figure, so you can let a friend borrow it, so he can claim it too. This way, a bunch of friends can get together and share their figures, so everyone can get a big selection quickly. Not to play with (other than on 3DS, as the 3DS let you store the characters in the game), but to help finish a collection.
8. To finish the games 100%, you will need one Skylanders from each of the eight elements, plus one Giant. (some chests in Swap Force can only be unlocked by a giant.) as well as one Swap Force character from each element. Keep in mind the swap force challenges are based on the loewr half of the character, so every top-half can do every challenge, if you add the right lower half. (this makes sense once you play the game.)
9. In other words, you will need a minimum of nine (9) figures to finish the game 100% (excluding the collections.) You will need one Swap Force skylanders from each element with their respective lower-half skill (teleport, turbo etc), as well as one giant.
10. So how many will you need to finish Skylanders 4 100%? If they don't add any new types of skylanders? None. Not a single one. If they add a single-element type like the Giants, you still don't need anymore. You get the last one in the starter pack. However, if they add more stuff like the Swap Force, you will naturally need one of those, too. But if they only expand the Swap Force with a few more lower half skills, you need those, too.
All in all, getting nine figures to finish the game 100%, but think of it as an investment. If you like the game and want more, you're near done collecting enough for that game anyway. If you're not sure if you will enjoy it, stick to the starter pack. You won't be able to finish it 100%, but there's still a ton of content here. :)
All this technology and gadgets is just a fancy way of talking to yourself. - Lara Croft, TR: Legend -