What are natural forms?
A natural form is when something grows or developes in nature. for example crystals are natural forms because they are formed only in nature.
No, neither Brachiomon or Cannondramon are the natural Ultimate and Mega forms of Armadillomon. According to the Digimon Adventure series, Cody's natural Ultimate and Mega forms for his Armadillomon would be Shakkoumon which is part of the Mutant classification and Vikemon which is part of the Beast Man classification.