? on Crochet Kit

Hi, I got a crochet kit as one of my Christmas gifts. There are 10 patterns for puppies but it says it includes everything I need to make 2. The patterns say to use a 2.5 mm crochet hook. A 3.5 crochet hook is included in the kit. I have a 2.5 mm crochet hook. Should I use the 2.5 mm crochet hook that I have or the 3.5 mm crochet hook that came in the kit when the patterns call for a 2.5 crochet hook?

The yarn that came in the kit is pretty thin. There's 4 colors of what looks like a 3 ply yarn. There's a couple white puppies but there isn't any white yarn in the kit. I have some white yarn but it's 4 ply or thicker. I might have some thinner white yarn. I'd have to look. But could I put a 4 ply yarn with a thinner yarn?

Besides the 4 colors of yarn they included the 3.5 mm crochet hook, safety eyes, black embroidery floss, a plastic needle and a small bag of stuffing to stuff the amigurumi.