Is a Leo Metal Rat and a Capricorn Fire Rooster compatible ?

Rat and rooster seems to be incompaticle. You may check https://www.travelchinaguide.com/intro/social_customs/zodiac/compatibility.htm
As to the Five Element, fire can melt metal. That is doesn't simply mean they are opposite. Earth is food for metal and wood is good for fire. Capricorn and Leo are not regarded as a good match by many people.