It's better to think of the Einhänder as a super fighter than a frigate. Even then, losing to a Cerberus - especially instantly - would be pretty bizzare. The only way I can imagine that happening is if it attacked the ship directly from the front without using its defensive system..and even then, i've never seen AI do that terribly when using it.
As for the Garegga, if you added the mod to an existing save, the only way you'd be able to come across it would be by finding the blueprint. In new saves there's a new system that's added with an independent market that sells everything.
This can be kind of a pain, so the next update will have them and most of the mod roster show up in independent + black markets at a reduced rate, while they wil appear at the mod added planet at higher rates.