My brother Thomas loves his orange fish named Lucas. The fish is only one, therefore the correct thing would be: "Lucas es pequeño". As the fish is male, it could not be "Lucas es pequeñA" because pequeñA is in female. As the fish is only one, singular, it is incorrect to say "Lucas soN pequeñoS" since there are no more fish next to Lucas. And the same with No. 4, you can not not only because they are more than a fish, but because it is also in female, and Lucas is a male fish.
