In order to gain access to the Side Story Chapters, you will need to follow these instructions:

• You must have less than 15 recruited units (which also includes the faceless units)
• On Chapter 24x Tiki must not have been recruit and Falchion is not acquired from Gharnef

So yes, if you want to access those Side Story Chapters, you will need to have less than 15 units. And by Chapter 24x, Tiki and Falcion should not be in present. In other words, if you want to gain access to the final Side Story Chapter, you must not have TIki and Falchion from Gharnef. But be warned, once you have Nagi to join you in your fight, Gotoh will not able to fight with you. But! As a Manakete, she is pretty much a powerful version of Tiki and can take down other enemy Manaketes as well. So use her to your advantage. Although, if you compare to Gotoh in this page, he is much more stronger than Nagi. So choose wisely if you prefer Manaketes over "Old men with ancient magic".

If you think it's not worth recruiting the characters in the Side Story Chapters, then you should not let your units die in the progress.