Is Wild Magic a Prerequisite for a Chaos Sorcerer?

Question

Is Wild Magic a prerequisite to playing a Chaos Sorcerer? If yes, when Wild Soul is used, how does the outcome affect the powers that already have a damage type?
Explanation : If I roll a 5 (Lightning), how will Burning Spray [At-Will ✦ Arcane, Fire, Implement] be affected?

Wild Soul : When you finish an extended rest, roll a d10 to determine a damage type.

d10	Damage Types
1	Acid
2	Cold
3	Fire
4	Force
5	Lightning
6	Necrotic
7	Poison
8	Psychic
9	Radiant
10	Thunder

You gain resist 5 to that damage type until the end of your next extended rest. The resistance increases to 10 at 11th level and 15 at 21st level. While you have resistance to that damage type, your arcane powers ignore any target’s resistance to that damage type up to the value of your resistance.

Also is it possible to play a Chaos Sorcerer using Dragon Magic?

Answer 1

Any character can be a "Chaos Sorcerer"

From PHB2, p138:

Chaos Sorcerer
...
Suggested Class Feature: Wild Magic
Suggested Feat: Arcane Spellfury
Suggested Skills: Arcana, Bluff, Endurance, Insight
Suggested At-Will Powers: chaos bolt, storm walk
Suggested Encounter Power: bedeviling burst
Suggested Daily Power: dazzling ray

Notice that all of those start with "suggested"? There is no such thing as a "Chaos Sorcerer" in 4e; it's not a mechanical term, it's just the name for a build that the designers thought was conceptually cohesive.

Remember that 4e explicitly allows players to reflavor their powers. As long as you don't change what the power does, you can describe it however you want. You can absolutely take Dragon Magic and make a "chaos sorcerer" by simply describing your spells as chaos energy (they'll still do whatever damage type they say they do, but they can look like whatever you want).

As for Wild Soul + Burning Spray, if you roll lightning for your Wild Soul and then use Burning Spray, your Burning Spray will ignore 5/tier lightning resistance on your targets. Which probably won't help much unless you've made it deal lightning damage (using a Lightning dagger or the Arcane Admixture feat, for example; not by reflavoring it, since that can't change damage types).