The VF-0S, SV-51, YF-19 & YF-21 where originally featured in a downloadable Bandai promo movie for the game. They were also mentioned on a few Japanese sites and also for a while on the Macross Compendium's info on the game.
However, shortly before the games release, all mention of these extra valks was dropped and for some unknown reason, the extra Valks were pulled from the game.
So basically besides all the VF-1 variants, the only extra Valk you can get in the game is the VF-4. IIRC, to get the VF-4, all you have to do is complete all the TV and DYRL missions (rank is not important).
We are all hoping that a PS2 Macross Zero and/or Macross Plus game will be released this year.
Graham