I'd definitely agree that trying to get finer grain than DDX with 5x7 film is a bad idea. With sheet film, anything up to say 400, you have virtually no grain to begin with, so the best thing to do is take advantage of that and go for the best tonality you can get, and also sharpness (though as a secondary consideration since there is not much that can make a print from a 5x7 negative unsharp). XTOL 1+1 or even 1+3 (depending on contrast) is slightly finer grained than DDX, but when I say that I'm talking about the differences you'd see in say an 8x10 print made from a 35mm negative. With sheet film the difference would be trivial.
DDX produces a relatively straight lined curve, similar to XTOL, D76 etc. It is slightly grainier but tends to give a well defined grain that has a bit more edge. DDX is also the "fastest" general purpose developer I've ever used (giving essentially box speed with Delta 100, for example).
It is quite a powerful PQ developer though, so development time and agitation can make significant differences in both tonality and graininess. This can make it "harsh" if not handled carefully. This could be compounded by continuous agitation, as is the case with rotary processes. I've found most published times for DDX too long, in some cases far too long. Experimentation is required.
Consider resistance to streaking too, as this can become a problem with some developers when rotary-processed.