Okay. If you don't show us your progress, most would assume that you are waiting for someone to do all the work for you. Regardless, here's my take on it.
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Standard Racial Traits:
Ability Score Racial Traits: "A Cambion is quick to act on their passion and uses an unnatural charm to still their whim. They gain +2 to Charisma, +2 to Strength, and -2 to Wisdom."
Type: Tieflings are outsiders with the native subtype.*
Size: Tieflings are Medium creatures and thus receive no bonuses or penalties due to their size.*
Speed: Tieflings have a base speed of 30 feet.*
Languages: Tieflings begin play speaking Common and either Abyssal or Infernal. Tieflings with high intelligence scores can choose from the following: Abyssal, Draconic, Dwarven, Elven, Gnome, Goblin, Halfling, Infernal, and Orc. See the Linguistics skill page for more information about these languages.*
*Taken from the Tiefling racial traits, using that race as a base.
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Defense Racial Traits:
The Cambion demon has two immunities; Electricity and Poison.
It has resist acid, cold and fire 10.
DR 10/Cold Iron or Good
and SR 17.
Also +6 Natural armor.
All these abilities would be broken on a PC, so let's water them down.
Immunities to Electricity and Poison -> Resist Electricity 5 and a Racial Bonus on saving throws against poison effects equal to your Hit Dice.
Resist acid, cold and fire 10 -> Resist 5 in two of these elements, chosen at character creation.
DR 10/Cold Iron or Good -> None at all. Not a good idea to grant DR.
SR 17 -> Same as above, not a good idea.
+6 Natural armor -> +1 Natural Armor.
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Feat and Skill Racial Traits:
Silver Tounged: Cambions gain a +2 bonus on Diplomacy and Bluff checks. In addition, when they use Diplomacy to shift a creature's attitude, they can do so up to three steps up rather than just two.
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Senses Racial Traits:
Darkvision: Cambions can see perfectly in the dark for up to 60 feet.
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Other Racial Traits:
Poison Use: A Cambion will never risk poisoning themselves when applying poison to weapons.
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Taken from the ecology of the Cambion Demon Bestiary:
"When an incubus mates with a human female, the offspring is a cambion. Cambions, unlike their “sisters” the alu-demons, are always chaotic evil and care little for anyone or anything but their own well-being. They are selfish, self-centered, and egotistical. Cambions hate humans and are often employed as assassins. Cambions are always male. Much like alu-demons, cambions are often sent to the Material Plane to seduce mortals or tempt them in some way.
The typical cambion stands 6 or 7 feet tall and weighs 200 pounds or more.
Cambions are deadly in combat, attacking relentlessly until their opponents are dead, often employing various poisons in battle to end the fight quickly. If combat goes against it, a cambion retreats, but never forgets the opponent that bested him. The cambion waits for the next opportunity to present itself so it might exact revenge."
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All in all, here's the RP breakdown, if you need it.
Type: 3 RP
Size: 0
Base Speed: 0
Ability Score Modifiers: 0
Languages: 0
Resistances: 3 RP
Silver Tounged: 3 RP
Natural Armor: 2 RP
Poison Use: 1
Poison Resistance: 3 RP
Senses: --
Total: 15 RP
(RP is a really bad way of estimating the power of a race, so you shouldn't pay that much attention to it.)
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This race should be about equal to a Tiefling in power.
Yeah, that would probably be because of the spell resistance. Spell resistance is VERY valuable, and no player race should have it for balancing issues.
Fiendish Resistance are nice, and I can see why you would boost the Electricity resistance to 10. But remember that Resistance 10 is as good as a Minor Ring of Resistance (with a price of 12000), and will block most electricity damage at lower levels.
And you shouldn't have resistance to all energy types either. Some sort of weakness/gap in your armor is needed.
You should settle for the normal fiendish resistance or just electricity 10.
Yes, getting rid of the Improved Resistances and Spell Resistance would greatly balance this race.
The greatest difference between the Cambion player race and the Tiefling would then be the following:
Tiefling:
Spell-like ability: Tieflings can use darkness once per day as a spell-like ability. The caster level for this ability equals the tiefling's class level.
Fiendish Sorcery: Tiefling sorcerers with the Abyssal or Infernal bloodlines treat their Charisma score as 2 points higher for all sorcerer class abilities.
Cambion:
Poison use and scaling Poison Resistance.
Natural Armor +1/+2.
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Now, a scaling Posion Resistance is weak at lower levels, but would make you effectively immune to poison effects at higher levels. This might be too good, so I think you should swap out the "scaling poison save" to a flat +4 bonus against poison effects instead.
If you do all this, then I believe the player race should be fairly balanced.
(Natural armor +2 is pretty good, but AC is much easier to gain than Resistance/Spell Resistance, so it should be okay)