Can you play as king mickey

In Kingdom Hearts 1, no.

In Kingdom Hearts 2, sometimes. If you get defeated in a certain boss battle, you will have the choice of playing at King Mickey for a brief period of time. Once in control, you can attack the boss or build up a charge meter which will fully revive Sora (think of it as an extra life). Note that King Mickey cannot defeat the boss fully; Sora must deal the final blow. Also, if King Mickey loses all health, he will automatically revive Sora to half HP.

In Kingdom Hearts Birth by Sleep, no.

In Kingdom Hearts 358/2 days, yes. He can be unlocked in story mode to be used in multiplayer.

In Kingdom Hearts Coded/Re:Coded, no.