I mix poly with cotton and different weights all the time. My bobbin thread is almost always lighter than the needle thread, but in the case of decorative work, you'd use a heavy thread in the bobbin and a fine/invisible thread in the needle. You just have to adjust the tensions properly for each thread. Set the tension on the bobbin for the bobbin thread, then adjust the tension on the needle thread to balance with the bobbin thread. It will be second nature before you know it. :)
Every time I buy a vintage sewing machine, at least one of the cabinet drawers is usually filled with old spools of thread. I keep the wooden spools and the Coats & Clark cotton and poly/cotton and other "good" threads and I throw away the styrofoam spools and brands that don't pass the break test. The Coats & Clark from the 70's and earlier usually chugs right along. :)