I ply commercial yarns all the time. I have an auto knitter sock machine and I got a good deal on yarn just five dollars a pound but some was very thin so I tried to ply it and it worked out great. I even took some thin cotton and Navajo plied it. It went threw my machine and made an awesome cotton pair of socks. So my answer is yes you can definitely ply commercial yarns.
great job!!!! Go to Menard's or Home depot and get Envirotex Lite. It is a two piece apoxy. I call it 50 first coats. Mix and pore over table. Spread it out with whatever you mixed it with. You will see some bubbles just blow on them and they will disappear. Now if you put a lip around the table it should be about 1/8 inch above the caps and you will have no problems the epoxy will just flow up to that, If not then you will have to let it drip of the side, make sure you take a brush or something so it covers all of the side, also wipe up the underneath so you don't have epoxy hanging down. I think you will only need the small epoxy bottles which i think were around $20.00 I just bought 128 Fl OZ and it was 50.00 dollars.