Babybear gave a great answer,

I'd also like to add that , remenber , it 's all math, governor and exciter models are just REALLYＲＥＡＬＬＹ　 ï½’ï½…ï½ï½Œï½Œï½™ã€€ï Œï½ï½Žï½‡ and repetitive math that simulates behaviour.

So to your original question " what is the rule?"

Start with the basics Governor IEEEST and exciter EXAC1
you can simplify everything to these 2 models if you ignore alot of stuff and you really wanted to

A analogy i like to use is , say i want to model your net worth
If you really wanted to you could sell everything you own and convert it to cash (if you really wanted to!!! )
however that is the worst way to do it.

REMEMBER IT"S ALL MATH!!