Or do as i do: First choose you legion, then recruit only those units that are in your legion`s upgrade tree. For example, you can choose Legio II Augusta or Legio IX Hispana. If you choose Hispana, you can recruit Legio XIIII Gemina, and if you choose Legio II Augusta, you can train Legio VI Victrix
Of course at start you must use weaker units, but if you use starting legion, you can just keep them in reserve and train them with you "trainer" skill