I don't think it's safe to assume that previous cycles looked character-for-character like Utena's cycle.  Nemuro's "cycle," if he had one, is a good example.  Based on flashbacks in Mikage's final couple episodes, we know that the other researchers expected there to be some kind of dueling game like the one in Utena's cycle (albeit there's no mention of a Rose Bride).  In Nemuro's case, though, that game was abortive; he "won" by incinerating all his competitors before any duels actually took place.  This stands in rather stark contrast to Utena's cycle, where she wins legitimately, and leaves less room for people to fill "Miki's role" or "Touga's role" in Nemuro's cycle.  Furthermore, as far as we know Nemuro/Mikage didn't face Akio at the Rose Gate or have his sword broken against it.

If Nemuro was a "previous Utena," the most interesting possibility is that Akio's enticements to burn the building down were the equivalent of his seduction of Utena.  Nemuro's test was whether he could retain his "nobility" under pressure of various kinds from Akio, Tokiko, and Mamiya; he failed.  Utena's test was whether she could retain her nobility in the face of seductions and betrayals by Touga, Akio, and Anthy; she passed, though Akio seems to have thought she failed.  This leaves open a number of questions, but character designs and thematic parallels clearly pull us to compare Mikage to Utena, so there may be something to this theory.

I'm less sure what makes Ruka a previous Miki.  The two have very different personalities.  Ruka is forceful, sometimes violently so, and doesn't mind making deals with the devil (Akio) for a chance at getting what he wants; Miki is pathologically restrained and explicitly refuses a deal with the devil (Mikage) because he doesn't know what he's signing up for.  Ruka was quite likely a participant in a previous cycle, but if so, I don't think he was that cycle's Miki or that cycle's Juri; he was Ruka.

As for why Akio would reuse a previous cycle's participants, given what we know of Akio's cunning, the real question is why he wouldn't reuse them.  Apparently he kept Mikage around specifically because he might prove useful as a test in some future cycle -- either that or we have to consider Mikage as a bona fide rival prince during Utena's cycle.  In any case, Akio isn't a guy who discards his pawns before they've outlived their usefulness.