Thank you for your post!
Unfortunately poly tubing fittings are very unlikely to be compatible with the PEX. Most brands will have a different inside diameter than the corresponding size of poly tubing. For example, a common inside diameter of 1/2" PEX tubing is 0.475", while a common inside diameter of 1/2" poly tubing is .600". The best bet would be to locate PEX specific fittings, they'll be designed to accommodate the inside diameter and (depending on fitting type) the thicker walls seen on PEX tubing.