Now I know that these are two completely different periods, but I was reading this book on Augustus' reign and it says that he created a sort of secret police to repress dissidents and political enemies, so I wanted to know how true this is and if so how similar this secret police was to the SS.
The pre-modern state would not have any where near the repressive capacities the modern state has. Pre-modern states, even tyrannical states, would be lacking in logistical capacity to conduct repression to the same extent that modern ones do.
An example would be how a secret police would function, in a modern state the secret police could be quickly deployed to a remote provincial town from the capital through railroad or automobiles. Once there, they could be instantly linked to the political leadership at the center through telephones or telegrams. The NKVD or the Gestapo would be excellent examples of this and would not be possible in the classical period.
In pre-modern times the government simply don't have the power to be totalitarian even if it wanted to. It doesn't have the resources or infrastructure to create such tools of repression even if wanted to. And to a large degree, it doesn't need to because pre-modern peasantry tends to be illiterate.
A pre-modern secret police is most likely to target certain elements of the political/commercial elite geographically located near centers of power. Modern states tend to be constrained by legal or institutional norms whereas pre-modern ones by their absolute capacity to act. It's quite anachronist to compare imperial Rome to Nazi Germany or the USSR.