Hola Jessica

I understand why you have your doubts on this. Ser and Estar are one of the most difficult topics to master when learning Spanish. This concept of "ser" for "permanent" and "estar" for "temporary" is a general concept that sometimes needs to be analysed a little bit more because it is a general explanation.

When you say "Tú eras bueno" we are still referring to a quality that that person had in the past. It was still a "permanent" quality then. This doesn't mean that they always had to be good and not change to "bad". 

The same would go for "Vosotros erais ricos": they were rich then (in the past, the time they're referring to) - the fact that they are now poor doesn't make us use estar. 

When you are talking about what you do for a living, you also use ser, as you mentioned, but I can change my profession and say for example:

Cuando era joven era maestra en un colegio, pero ahora soy enfermera en un hospital.

When I was young I was a teacher in a school, but now I am a nurse in a hospital.

I'm still using ser for both, because those were seen as "essential characteristics" of me then and now. (I prefer to say essential characteristics better than permanent)

I hope this showed a bit more light on the subject.

Saludos cordiales

Inma