I usually use the same kind/type of thread in the top and bobbin threads because I think that "the same" will be easier to sew with as far as tensions go.
I have not done any experiments with "combinations" - so can't give any input on what might happen.
I think - when people do embroidery - they use two different kinds of threads a lot of the time.
If the stitching looks like you want it to - I am reasonably sure the sun will come up again the next morning regardless of what kinds of threads you use - and your seam will hold together, too.
Last edited by bearisgray; 10-01-2015 at 04:26 AM.