You can use whatever you want, or throw in a pint of oysters, add a pound of firm white fish fillets, or use chicken instead of andouille...or leave out any other meat than seafood. Gumbo is as forgiving as any other stew, as long as you start out with a decent roux. The trick to roux is low and slow...if the heat is too high, you'll scorch it and then it's good for nothing but the trash. You can also add as much or as little of your favourite hot pepper sauce (I usually toss in some Tabasco toward the end) depending on how hot you want it. If you're serving folks who like thicker gumbo, pass additional file powder at the table for them to mix into their individual bowls.