Here's how slave attacks work. If you just did a melee or bullet attack, and are holding 6, the slave will execute a tag-attack on swap-in; there is a special ground melee, air melee, and bullet tag for each slave. (The ground melees pretty much all hurl the opponent into the air, which can be somewhat inconvenient) Holding 6 and tagging during your spellcard also allows you to double-spellcard.
If you're doing a C special, you can tag into ANY of your slave's C specials by holding the appropriate direction while tagging. Or no direction, for 5c. So you can totally tag from 2C into 6C or whatever.
Generally, you can cancel your attacks with a tag-swap, so you don't have to wait 'til they're over. However some attacks will require hitting the opponent (or getting blocked) before tagging out is allowed.
By the way, it costs 50% of your tag meter to tag into attacks, or to tag out of blockstun. (As the slave, your meter will slowly run down 50% instead of instantly dropping) If you don't have at least 50%, you can't do it. Also, tag-swap combos do much less damage when your meter is low. There's no damage penalty on attacking as the slave outside of a master-into-slave tag combo, though.