Sing is used to sleep the opponent, and is considered by some to be better than Thunder Wave, due to Electivire's ability to speed up when hit by Electric attacks. It also is able to affect things like Garchomp, disabling them when Thunder Wave could not. No Pokémon bar STalkers like to be asleep, while some Pokémon are fine with Paralysis. There are some disadvantages to this, such as the aformentioned poor accuracy, Sleep Clause, and STalkers, but there are also disadvantages to Paralysis, and it seems to come down to personal choice. Don't quote me on that though.
Intimidate is generally better on Arcanine, though if you want to bait Fire moves with Forretress, that's fine, though you might want to run Sunny Day as well for massive Fire damage.
I believe Choice Scarf multiplies Speed by 50%, equal to one stat boost. I'm not certain however.