when making amigurumi, i always use a smaller hook than recommended. for red heart or most standard weight acrylic or cotton i use a 3.50 mm hook. it keeps the stitches tight, but not too tight. if the stitches aren't tight enough, the stitches will stretch and expose the polyfill/stuffing material, which i find unsightly.
good luck with your first project
this is very true....but the yarns you mentioned will work fine...i ALWAYS use an acrylic yarn from ami's unless I need a different type to effects like fur, or feathers.