The ships in rtc_shiparch are the ones used in the SP story cutscenes - I´m pretty sure RTC stands for real-time cutscene. None of those currently in the file can actually be used as ships, they´re just used as props that are moved around the cutscenes.
And... if you want custom ships to fly around your cities... you need to put the reference to the ships´ CMPs and MATs into rtc_shiparch.ini to see them there. The only bummer (from my POV) is that rtc_shiparch.ini doesn´t appear to allow you to attach Equipment to a ship... so you won´t see Engine glows or other attached things.