That could work, but it would also put the player in a non-neutral starting position which isn't particularly good when looking at the big picture of making choices about allegiances.
It would be better, if the player was opted at the beginning with - say - three predetermined characterclasses with predetermined allegiances and one random class. The predetermined classes would be tailored to suit their starting position (skill/statwise) while the random class would be player created. All of the 3 predetermined ones would have their own distinct benefits and disadvantages over the other classes (including the random one).
For a very crude example:
Class 1: A scientist build - specialised in - you guessed it - science, with moderate beginning knowledge of, say, energy weapons and speech. And a science oriented special ability (like being able to operate with higher level scientific equipement at lower level than others, but starting with inefficiency in combat). Starts with (for examples sake) Followers of the Apocalypse, with the game reflecting that starting position and allegiance.
Class 2: A soldier build: Specialised in combatative abilities, inefficient in social and scientific skills. Special ability (lower requirements in weapons and lesser encumbarance effects with armor, but inferior in science and social skills). Starts with (just an example) BoS, with the game reflecting that starting position and allegiance.
Class 3: A trader build: Specialised in social skills, with average knowledge on conventional firearms. Special ability (lower speech thresholds and lower barterprices, but overall carryweigh is much lower too). Starts with (example blabla) Crimson Caravan, with the game reflecting that starting position and allegiance.
Random Class: Playercreated, no special abilities. Starts at predetermined neutral location on the map.
Bethesda ain't Bioware. If you want predetermined storylines or classes go play Mass Effect or Dragon Age.
The beauty of Bethesda's system is you can create your own storyline for yourself. Having this kind of an explicit build doesn't really add any value imo.
The Tag! and skill system already allows for a huge amount of builds. In New Vegas alone I have 4 unique builds for all styles of play;
Conman/Gambler; Speech. Barter. Lockpick.
High Luck and Charisma. Low Strength and Endurance.
Rakes in a fortune with Casino Blackjack. Earns bigtime with high barter, gets better rewards with Speech.
Morally grey, tends towards selfishness.
Legion Assassin; Melee. Survival. Sneak.
High Strength and Agility. Low Charisma and Luck.
Survives in the wastes, uses throwing weapons, daggers etc...
Cannibalistic when needed, Mr. Sandman for assassination missions, infiltrates the enemy and takes them down when least expected.
NCR Agent; Guns. Sneak. Speech.
High Agility and Perception. Low Stength and Endurance.
NCR man through and through. Assassinates the Fiend leadership, takes down the BoS, kills the Great Khans.
Sneak and Guns build, ninja perks, better criticals perk, etc...
Scientific Altruist; Energy Weapons. Science. Medicine.
High Strength and Endurance. Low Agility and Luck.
Independent technophile. Helps the poor and those in need.
Power Armor and Energy Weapons build. Pro-BoS. Heals those in need, helps the Followers of the Apocalypse etc...
Bethesda has allowed many situations where I could make choices integer to my builds;
Helios One, Brotherhood Quesline, Crimson Caravan Line, Kings Questline, Nelson vs. Forlorn Hope, etc...
No need for explicit character build choosing, you can choose your own path and stay true to it pretty much the entire game.
Though, if you really want an explicit choice at the beginning, I can recommend this mod:
http://www.newvegasnexus.com/downloads/file.php?id=37042