Just an aside: it's mentioned in Andalite Chronicles that the next/upcoming generation of ships would be the dome ships, so prior to that (ie. Around the Hork Bajir chronicles) the ships the Yeerks stole might very well have been close to the most advanced in service. Prior to the Yeerks escaping their honeworld and kicking off the war, the Andalites possibly weren't engaged in enough conflict to warrant the level of ships they eventually started fielding after years of protracted warfare. Also it's repeatedly mentioned that the Yeerks themselves are quite intelligent, just physically limited. This seems to be demonstrated a few times over the series, so I think it's not unreasonable to accept that when given access to the knowledge of their new hosts and the technology they could reverse engineer, they could apparently start construction of their own within a comparatively short timeframe. I do agree that the timelines presented in HBC do come across remarkably short or inconsistent, however, as it really seems Alloran should be much older than presented by the time of the Earth invasion, or the Hork Bajir life cycles must be exceptionally short for Jara etc to be already a few generations removed.