Is there science to lifestock timelines?

I'm so new to this hobby I'm sure I'm asking stupid questions.

I'm sorry about that, and I do try to search before asking :)

So... is there a science to the time between livestock species?

I've read you need about 2-4 weeks (and my plan is 4 weeks between species), go most docile to most aggressive, etc

I'm assuming the 'most docile to most aggressive' is so the docile ones are fully acclimated before anything more aggressive comes into the tank (though most of my livestock will be docile to start with).

I'm also assuming the time between species is to allow them to get used to the tank (acclimate) and to allow the tank to adjust (bioload, natural bio filters) before adding a new species.

Is there a science to the timing? I like 'rules of thumb' as they are very helpful, but what is it based on?

The tank needs roughly 4-6 weeks (sometimes less or more) to cycle, but we can test to see when it is actually 'cycled'

Is there tests or behaviors that say 'yup its ok to add the next species'?