October is the tail end of the rainy season, which means that sometimes the water can be too high (depending on which rivers you're looking to hit, and how much you want to take on). It could be fine, but rain can bring up the river to pretty scary levels. In the Valles area, the Micos waterfalls turn into huge terminal drops at high water w/ backcut caves behind just about every drop. The Santa Maria would be like jumping into the Upper Taos Box at 2000+, and the Verde would be huge and some drops would be hard to portage. The Salto is dam-released and would probably be fine.
I think that generally you would probably be OK for flows about that time, but it's hard to say- if there's a monsoon kicking throught Valles (or worse, a hurricane) you might have to drive further down to the Veracruz area to find rivers that are flowing at a more sane level. Just something to consider, since it's nearly impossible to find out what the rivers are running before you get down there.
I'm probably going down for two weeks in the middle of November this year- that seems to be a good balance of higher flows without too much rain.