Making movies from the Chronicles books is something you do AFTER the show is a success, not before. As good as the Chronicle books are, they are dependent on the things we learn in the Animorphs series proper in order to retain meaning. You need to have a steady preconception of Elfangor's character before the Andalite Chronicles, otherwise learning the Elfangor is NOT a hero doesn't have nearly the same impact. Likewise, the Hork-Bajir Chronicles rotates at least partially around our preconceptions of Visser Three and Alloran.
And that's just how they function as books. In a television medium, if you had one of the Chronicles play first, then viewers going into Animorphs, being advertised as a continuation of that neat alien movie with all the weird visuals they saw, will be treated that is visually, tonally and thematically very different, and it may turn a lot of them off. No, best to start off with the main series.