. . . . . Then, all by itself, the sentence you mention. Is it referring to all of the above, or does it stand alone and refer to reciprocating engines as well?
If the latter, then can you run a XF,XXF, XO engine in Classic?
Honest answer is that I really don't know for sure. But it appears to me that the only part of the new rules that applies to Omega and is the stuff about swept volume factor to adjust the 'displacement' of rotaries. BTW the factor in 2010 was (is) x3.
So I think the sentence . . ."In classes where not all engine breaks are available, the smallest displacement class allowed is open to all engine displacements that fall within it and below it." applies to all engines, recip, omega, rotary, whatever . . .
I do not think that this implies that XF, XXF, XO, XXO or V4 or even V4F can run in the smallest displacement class. Somebody correct this if I am wrong, but, I thought the the vintage engines were always allowed to run in Classic but they run in the class that matches their engine displacement. Example - I think a 74 Mustang gas coupe with a 239 cubic inch flathead V8 engine would be in E/CGC and not XF/CGC. Why not F/CGC you ask, . . . because a 239 cubic inch engine is bigger than "all engine displacements that fall within it and below it." where for Classic 'it' is F class with a maximum engine displacement of 183.99 cubic inches.
Humm. . . I just re-read what I have typed above and I can probably say for sure that this is an example of why I would be the wrong guy to re-write the rules. Heaven bless those willing to be rules writers.