The 20% should be fine initially, after a few weeks you might want to find a feed with lower protein or cut it with wheat or another grain to reduce the protein. I lent my book out so I don't know what the recommended age is for reducing the protein but SGTRD would provide a recommendation. I actually start with a gamebird starter and switch to a custom ration throughout the brooding stage.
A heat lamp is needed as long as the birds need it. I don't ever use a set number of weeks or put a heat lamp so that it reads exactly 95 degrees for exactly one week, etc. I change the wattage of the bulb as needed, and I keep it on them as long as they need to have it. Even after, sometimes, as the light at night attracts bugs so they get a buffet each night!
Your ducklings will tell you whether they're too hot or too cold or if they're content. The best gauge is their body language. If they're huddled together in a pile - they're too cold. If they're spread out and panting - too hot. If they run around and play & eat/drink and are relatively quiet & spend some time napping peacefully, they're content. Unhappy babies get loud! I've had ducklings/goslings that were 2 weeks old that were perfectly content without a light during some really chilly rainstorms. They were able to get out of the rain & get dry and they were perfectly fine even though I made sure to check on them a lot. Had they told me they were cold, the light would have been turned back on.
