Some good stuff so far. I'll add my twist (templar style)
I think it depends on what the purpose is.
Long range shooty squads, 6 man las/plas or plas/plas.
Short range shooty squads 8-10 (drive-by squads or drop pod squads)
Though we do not have them, you codex players do. 10 man with heavy weapon, special weapon and vet sgt with fist. Good for what ail's ya. Can shoot from afar, can move in close and rapidfire and can also do well in an assault do to the marines toughness and the powerfist.
Any squad designed for an assault roll should be 10+. Either footsloggers, rhinos squads or assault squads. Though assault squads of 8 probably would not be bad in smaller games or if you want to give them more tank hunting type roles. I would keep them 10 strong and use them to crush the enemy.
Assault: 8 is good 10 is better, two squads of 8= 16 is best two squads of 10 = 20 is DEATH
A most excellent idea. They move together, they support each other and you are pretty much gauranteed at least one squad will make it anywhere at or very near full str. For extra assaulty goodness add in a chappie with a JP. These three units will destroy things. Anything.
For devs I say 2:1 marine to heavy weapon. We don't get them to play with, but I drool over them all the time anyways.
Shooty termies are fine at 5 man. 6 would be nice for VPs, but 40 points is alot to scrape up. More than 6 is useless. Just make another squad so you can get 2 more asscannons!
Assault termies. I dunno. I have never used them.
I don't get scout squads, either, but same ideas as behind the tacticals. Big assaulty, med to big for shooty. My fav squad before the new Templar codex was 5 scouts with 2 bolters, 2 snipers and 1 heavy bolter. It was cheap, but decent firepower.