is the outline supposed to be done in satin stitch? or in some other decorative manner?
if that's the case, 30 wt thread might make a stunning impact (if you can find it.) you would need to use at least a size 16 needle.
i agree with those who advise you to not worry. if you can't find 30wt thread, just use what you can find. 40wt should be a bit easier to track down. 50wt is very easy to find. just remember to use the correct needle size and it should be just fine. :-)