re: Seafood stock recipe for shrimp gumbo (recommendations)

Most "rules" of stock apply to restaurants, which use stocks in multiple ways, not known in advance.

For example, restaurants want stocks to be clear so they can be used for consomme, or dishes like that. But since you're making a stock for gumbo, clarity is of no concern.

If you have fish heads, use them, they have tremendous flavor. But first remove the gills, which are bitter.

Nothing is to be gained by cooking fish fumet at high heat, or for a long time. Cook it at the gentlest simmer - 180 degree F is enough.

30 minutes is plenty of time. If you want to flavor it or reduce it further, strain out the bones after 30 minutes, then add your veg and boil away.