You should have no trouble getting the Class 1 with a cat/dust mite allergy. But don't bring the cat while flying! As long as the allergy is under control and you don't have to use medicines for it constantly you won't have a problem. I would let the AME know though, it's better you tell them and be honest about it. Discuss it with your AME, he/she might even give you tips on what allergy medicine that can or cannot be used while flying.