Big question, then, is whether the 700 rear tire that you want would fit in the frame?
You aren't asking if 26" and 700 tires can be used interchangeably on either size wheel, are you?
Best plan would be to start with a frame designed for the size wheels that you would like to use.
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Bacchetta Giro A20, RANS V-Rex, RANS Screamer