I'd say yes, its been 50,000+ Years and many species were either completely destroyed or relegated several hundred years in tech. Thus, it isn't implausible someone showed up and kicked their arses

I'd say a dozen or two dozen at most and a few hundred ships, we will get to the hundreds of planets and a thousand ships later. The idea is that everything has a value, so we cant just throw away massive fleets or just abandon planets for no reason. A blow will actually hurt us instead of being "Ah! But I call up my six billion large reserve fleet and crush you!"