@Matt I'm surprised you have been diagnosed with lymphedema before surgery. Have never heard of this. In any case I'd stick with the sleeve since this is important right now before the condition gets a go on. Try to access a physio who specialises in lymphedema treatment and ask for base arm measurements to be taken. This way you can benchmark future swelling, should it occur post surgery.
Dont worry too too much about having the cancer in you while chemo is going on. Even if it doesn't kill the cancer, it will get it under control so the surgeon gets a clear run at cutting it out of your breast and your lymph nodes. Likely they will remove all these and that's when your lymphedema risk really is a greater risk and the sage advice from @Afraser really kicks in.
Keep us posted as things develop.