as far as AC.... I would make any bonus small and heres why...they are giants to begin with so they would have the lowered mundane damage, that accounts for scales I would think. Top that with Reccums suggestion to add similar magic weapon resistance due to their "magical" nature, and the AC will not need to be great. 25 con for fifty levels and resistant to all weapons save piercing and the slayer should sufice. The ONLY objection I have is this, say your zerker dragon cleaves my dragon slayer, suddenly the playing field is no longer even and never will be again between you and I. So maybe we could add a single "element" vuln to each breed of dragon to maintain a level playing field eternally.
Project Dragon
Or we could make the dragon slayer burnproof.
I dare anyone of you to cleave a Dragonslayer of mine.
I even had one that was double blessed.
lol Mya.
If someone cleaves a weapon of yours thats only your own problem tbh.
It is dragons actually that should be worried about someone cleavig or destroying their weapons because they will not have access to this burnprooving "technology" 
As far as their scales adding some sort of physical immunity - I somewhat think that magic resistance + phyisical immunity might be a bit too much.
I dare anyone of you to cleave a Dragonslayer of mine.
I even had one that was double blessed.
Ooohhh, double bless? Ah ha! You must have been a cleric! 
I dare anyone of you to cleave a Dragonslayer of mine.
I even had one that was double blessed.
Was I the only one who read this and had ominous-but-cheesy music go through their head with it?
Barely seventeen and barely dressed? JK I know I am old.
Question/s: Aren't dragons an ancient and noble race on their own? What would be the RP behind a giant "turning" into one? And why only giants? If it is some kind of mystical curse/blessing, I don't see why it would be exclusive to just giants (that are typically dumb and very unlike dragons intellectually).
I will answer your question with a question of my own - do you have an idea for other race who would fit the RP better?
They might be dumb, but they are the closest when it comes to physical appearance to dragons. Only demons get closer.
Or sliths
I am sad to say that I have had a few dragon slayers cleaved.
shakes fist at balinor
Or sliths
You are right actually. Sliths are a good alternative too.
I originally wanted them to be warriors and zerkers though. A dragon blademaster or a thief might be a bit too OP.
I figured they would be their own class.
Their own class makes sense.
I would be tending to Human as base race. Or any race.
You scrap evolution to a Dragon, and have the human RP as a dragon hunter. When the APP is approved, you can have a dragon come in and eat the Dragon hunter.
And then you start to control the Dragon, as you now are the Dragon, and you old character is dead.
howabout they need to pull off the rp/and everything else required to be a restrung dragon first? :-P
Their own class makes sense.
I would be tending to Human as base race. Or any race.
You scrap evolution to a Dragon, and have the human RP as a dragon hunter. When the APP is approved, you can have a dragon come in and eat the Dragon hunter.
And then you start to control the Dragon, as you now are the Dragon, and you old character is dead.
Leaving the comical part aside, there is no need to restrict RP to one path only.
There are so much possibilities for dragon RP.
Lets talk mechanic wise.
Your race is to overpowered.
You want remorts? OK lets allows 50's remorts, but I hate them. I would prefer remorts to happen at 30.
Let's not create a Dragon class, as this is to much trouble for now.
Bashing classes cannot have size higher than Giant.
And you cannot have to many bonus. Magic resist,Giants,Breathfire.
So let me compile your idea into a new (my) idea.
Remort. Same class. New Race.
Races for Remort: ANY
Classes: Warrior, Zerker, Cleric(Healer,Shaman), Invocker, Battlemage. (No DK because to powerful, so Paladin out to.)
Race Stats: High STR, High CON, Giant size.
Rank 1 STR 20 INT 20 WIS 20 DEX 20 CON 25 Size Tiny
Rank 10 STR 21 INT 21 WIS 20 DEX 19 CON 25 Size Small
Rank 20 STR 22 INT 22 WIS 20 DEX 18 CON 25 Size Tiny
Rank 30 STR 23 INT 23 WIS 21 DEX 17 CON 25 Size Normal Auto-Fly
Rank 40 STR 24 INT 24 WIS 22 DEX 16 CON 25 Size Giant Auto-Fly
Rank 50 STR 25 INT 25 WIS 25 DEX 15 CON 25 Size Giant Auto-Fly
This simulates Dragon Growth.
Giant at 50. Scale means average 22.5 INT/WIS for mana gain. 25 CON to offset vulnerabilities.
WIS scales to prevent 3+ prac per level.
Dragons are natural magical beings. So they have natural spells (CRAP SPELLS for RP)
Chill touch,Flaming Hand,Shoking grasp.
L30 - Lightning bolt.
L40 - Magic missile.
Vulnerabilities: PIERCE-(ALL) COLD-(EVIL) LIGHTNING-(Neutral) CHARM-(Good) WATER-(Good)
Ressistences: MAGIC
SKILLS: (Natural Dragon skills)
@10 Scales - Ability to harden Scales. Affect AC by 2x -Rank (At 50 -100 AC).
@20 ROAR - Zerk ROAR
@30 BREATH-FIRE (As in game) FIRE-(Evil) POISON-(Neutral) COLD-(Good)
@40 Enlarge for 1-2 hours, 24 hours Cooldown. (No stacking with Enlarge spell.)
@50 A skill that allows you to appear as someone else. Size to normal. (Shut up)
Colours:
Red-(Evil) Green-(Neutral) Blue-(Good) (Slith colour)
Pondering addition Restriction: RARE LIMIT.
If you have diferent ideas for Neutral/Good Vuln and breathfire or colours sugest.
Hoo, I scratched your Malform powers Idea. Lets not promote more mindless RPless PK.
If you want it that much, just make Breathfire Power like Ray/Path. Based on PK record.
Evil Dragons - More Kills more damage.
Good Dragons - More kills by oponent = more damage.