Some optimization, since the other answers cover the negative case well...
For stealth, consider "The rules of hidden club" as a fantastic discussion on how stealth actually works in D&D. You also cannot make a stealth check to hide at the beginning of an encounter because you're not a gnome nor have you taken a move action.
The trick you're looking for is shadow walk + cunning sneak for perma-stealth. Or, to allow for your specific goal, shadow walk + hidden sniper give gives you CA any time you have concealment.
You can get cunning sneak through hybrid talent, and shadow walk through accursed shadows. You may also want to consider the level 10 utility persistent tail.
Edit: Working off the "how can I optimize this" part of your question, I recommend:
====== Created Using Wizards of the Coast D&D Character Builder ======
Drisdahun, level 6
Pixie, Rogue|Assassin
Fey Beast Tamer Starting Feature: Fey Beast Tamer Companion (Young Owlbear)
Hybrid Assassin: Hybrid Assassin Will
Hybrid Talent: Rogue Tactics (Hybrid)
Rogue Tactics (Hybrid): Cunning Sneak (Hybrid)
Background: Silent Hunter (Silent Hunter Benefit)
FINAL ABILITY SCORES
Str 8, Con 13, Dex 20, Int 10, Wis 10, Cha 18.
STARTING ABILITY SCORES
Str 8, Con 13, Dex 17, Int 10, Wis 10, Cha 15.
AC: 21 Fort: 15 Reflex: 20 Will: 19
HP: 44 Surges: 7 Surge Value: 11
TRAINED SKILLS
Perception +9, Dungeoneering +8, Streetwise +12, Stealth +16, Thievery +13, Bluff +12
UNTRAINED SKILLS
Acrobatics +8, Arcana +3, Diplomacy +7, Endurance +4, Heal +3, History +3, Insight +3, Intimidate +7, Nature +5, Religion +3, Athletics +2
FEATS
Level 1: Hybrid Talent
Level 2: Student of Malediction
Level 4: Cursed Shadow
Level 6: Ki Focus Expertise
POWERS
Hybrid at-will 1: Sly Flourish
Hybrid at-will 1: Executioner's Noose
Hybrid encounter 1: Shadow Darts
Hybrid daily 1: Targeted for Death
Hybrid utility 2: Cloak of Shades
Hybrid encounter 3: Low Slash
Hybrid daily 5: Compel the Craven
Hybrid utility 6: Pixie Invisibility
ITEMS
Bracers of Archery (heroic tier), Scavenger Bird Ki Focus +2, Hand Crossbow, Shadowdance Leather Armor +1, Collar of Recovery +1, Adventurer's Kit, Rapier
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The first mistake is combining melee and range. While it works in theory, it's very hard to pull off in practice. The cunning sneak, shadow walk combo is solid, though a touch gimmicky.
Here, you flit around the edges of the battlefield and drop sly flourish while you build up sufficient shrouds on your primary target, then you drop your nova-encounter.
Ki focus powers all of your attacks so you save on implements. Pixie flight will come in real handy and be a fascinating way to play through the game. As a ranged scout/striker who is tiny, you'll have all kinds of out-of-combat fun as well as in-combat. At the same time, your fey beast will provide a meatshield in combat so that you're not placing an undue load on your defender or leader.
DPR of this build:
`Sly Flourish: +13 v. AC / 1d6+13+2d6 = 80% accuracy, .75*(d(3,6)+13+2)+.05*(3*6+13+2+d(2,6))=21.12 = 3.40 round striker. Not particularly good, but above the 4 threshold so it works, especially with the stealth assets you bring.
And then you can dump 2-4 shrouds whenever appropriate on another mob, and then open as appropriate.
With more thought, I realized we were overthinking this. In the build above, you were using this whole complicated edifice to power stealth for combat advantage.
With the thief guide:
====== Created Using Wizards of the Coast D&D Character Builder ======
level 6
Drow, Thief
Fey Beast Tamer Starting Feature: Fey Beast Tamer Companion (Young Owlbear)
Background: Born Under a Bad Sign (Born Under a Bad Sign Benefit)
FINAL ABILITY SCORES
S tr 8, Con 13, Dex 20, Int 10, Wis 10, Cha 16.
STARTING ABILITY SCORES
Str 8, Con 13, Dex 17, Int 10, Wis 10, Cha 15.
AC: 22 Fort: 15 Reflex: 21 Will: 17
HP: 57 Surges: 7 Surge Value: 14
TRAINED SKILLS
Stealth +15, Thievery +13, Athletics +7, Acrobatics +13, Bluff +11, Perception +8, Insight +8
UNTRAINED SKILLS
Arcana +3, Diplomacy +6, Dungeoneering +3, Endurance +4, Heal +3, History +3, Intimidate +8, Nature +3, Religion +3, Streetwise +6
FEATS
Level 1: Crossbow Expertise
Level 2: Backstabber
Level 4: Ruthless Hunter
Level 6: Two-Fisted Shooter
POWERS
Lolthtouched: Cloud of Darkness
Thief utility 1: Tactical Trick
Thief utility 1: Acrobat's Trick
Thief utility 2: Fleeting Ghost
Thief utility 4: Sneak's Trick
Thief utility 6: Chameleon
ITEMS
Rebounding Hand Crossbow +2, Magic Leather Armor +2, Bracers of the Perfect Shot (heroic tier), Hand Crossbow, Amulet of Protection +1
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We get what you were working towards. First, with tactical trick, you get combat advantage against... anyone adjacent to an ally. You also get your full move with limited OAs, so you'll probably be able to find cover. With crossbow expertise, you ignore cover and superior cover and with two-fisted shooter you reload as free.
Therefore, every turn you can attack with CA, not dependent on a stealth roll.
Your DPR, counting your owlbear (who counts as an ally, BTW, and who can move when you take a move action, meaning you can target anyone you like with tactical trick): `+14 v. AC, 85% accuracy (counting perma-ca), 1d8+11+2d8 damage. with free RBA on crit) = 24.80 DPR. 2.90 round striker which puts you into the big leagues. With 2 uses of backstab and one rebound you'll hit every single turn (almost), and your DPR creeps ever higher.
You also get the stealth advantages from your build from sneak's trick, allowing you to sneak through deserted corridors (counts as cover) with impunity even if one enemy sees you because of chameleon. The melee version of this is even better due to the charge chassis it is built upon, but it doesn't seem the build you're trying to go for (and is sooo bloody boring).
Best Answer
This is a great guide to all things hiding and hidden in 4e.
The gist of it is that you need Superior Cover or Total Concealment to become hidden.
Superior cover is defined as 3 or more lines from corner to corner from your square to the target square are obstructed. Total Concealment is things like invisibility, dense fog, or total darkness.