I use both silk and cotton to hand appliqué. I prefer the silk as it is so fine the stitches hide easily and I only have to use a few spools. Needles are a matter of preference. I have used straw, appliqué, and betweens. If I have a lot of sharp corners I like longer needles. Otherwise i just grab whatever is handy. For now go with your teachers suggestions. This is the voice of experience and it sounds like she will go with cinventional wisdom.