There are rules about Cajun gumbo that have been passed down through the generations. Like all commandments, they are sometimes broken, but with repentance, these transgressions can be forgiven.
1 Thou shalt never use tomatoes in Cajun gumbo. The Creoles sometimes slip a tomato product into their gumbo, but Cajun gumbo hath no tomatoes.
2 Thou shalt not use Un-Cajun Sausage. Louisiana makes plenty of sausage. Andouille and smoked sausage work just fine. The one exception may be Conecuh sausage. It's made in Alabama but tastes like it is made in Louisiana.
3 Thou shalt not use store-bought roux. Just don't. Take your time and pride of craft to make your own roux.
4 Though shalt always use a bowl. Don't put gumbo on a plate. Only Philistines and Yankees would commit such a sin.
5 Thou shalt only use a wooden spoon. When you're cooking roux or assembling the gumbo, a wooden spoon is an absolute necessity. We might use a metal ladle to dip it out for serving, but the wooden spoon is critical for a good roux.
6 Thou shalt always adhere to the "gumbo to rice" ratio. It's simple match. Two parts gumbo to one part rice.
7 Thou shalt always have filet available. Filet, or ground sassafras is a gumbo staple. Some folks like it, some folks don't. But it is a sin to make gumbo and not have filet available for those who like it.
8 Thou shalt not mix chicken and sausage with seafood. Just don't. If you want a seafood gumbo, make it. If you want a squirrel or a duck gumbo, make that too. If you use catfish, it is called a court-bullion.
9 Thou shalt not tell people where the potato salad goes. Some like it in a separate plate, some like it in their gumbo. It's personal taste, and none of my business. Nor your business either.
10 Thou shalt always stir counterclockwise. I don't know where this one came from, but I always stir counterclockwise. It just always been like that. I'm sure it is a minor transgression and has no detriment to the gumbo.
If you get down to New Orleans, all these rules might not apply, but the Creoles have been thumbing their noses at the Cajuns for years. But, just because someone ignores the rules don't mean that the rules don't apply. Repent and prosper.