[kazana]

A nice sidekick combo is
defense: dry bones
wings: toad & hammer

dry bones is nice and fast, which is essential for defense. he's also got an acceptable attacking range which you'll need in the back. and if you happen to have him with a charged ball, you can just sidestep the defenders, get somewhere near the goal and roll him past any goalkeeper.

toad is also fast, but his attack range is very small. I mainly use him to jump over the goal keeper for an easy score. it takes some practice to get the timing tight, but it's really easy.
if you struggle with the timing, just jump into the keeper and pass the ball before the keeper hits you, then your other players can sink it in the open goal.

hammer is mainly there for distance shots. charge the ball, pass it to hammy, and whack it. don't be shy to release if someone is blocking, since that charges the ball even more (plus you get items).
bird should do just as well. main thing is, with two fast/tricky sidekicks you'll need one with serious shotpower.

as for captain:
i've been using yoshi, but as stated by others before, his special abilities suck monkey balls. i'm probably going to start over with DK, because in the striker cup you really need a deke that will knock others down - yoshis dumb hobbling thingy is plain useless, and his special ability ain't that much better either.
and another stong striker would really help for those teams with fast defenders where you just can't get close to the goal. charge the ball pass it to DK or hammy and whack it. that's the idea.

what annoys me most about the striker cup (where i'm stuck with yoshi and team) is that whenever other captains start to pull off a megastrike their sidekicks will simultaneously knock over any player close by. highly annoying, i wish i was able to do the same. argh. i haven't gotten past the semis there with this team yet.