With Dark Angels, your troops choices are limited to Tactical Squads.
That's ok, though. Customizing a Tactical squad for your personal needs is super easy. Especially due to the 'Combat Squads' rule.
Basically, this rule means that at any time during a game, I can split the Tactical squad into two 5-man squads. Doesn't sound like much, but it can be pretty useful. Imagine giving your veteran sergeant a pistol/melee weapon, as in the pic above. Now, I also want a Plasma Cannon for some added punch. Now comes the question of "Are you getting the most out of your equipment choices?" By equipping some with assault-oriented weapons, and some with Heavy weapons that restrict move-and-fire options... you cut the efficiency of your squad.
Well, the easy answer to that is to break the squad up into two combat squads. Have the Heavy weapon chill out with 4 bolter-buddies ... meanwhile freeing up your assault oriented veteran sergeant and any special weapon (flamer, plasma gun, meltagun) to go charging off to wreak some down-and-dirty havoc.