I don't think you should use PF monsters. My understanding is that their CR is a bit closer to good. Here's a source for 3.5 monsters, one that can be sorted by CR. Should do the job just fine, and isn't assailable on the grounds of completely different sources. If you want to use only some monsters out of the full set, I'd just use this random number generator, with the total number of monsters listed at the CR as the top end. For example, CR 5 has 107 monsters listed, and I got a 48, so that'd point to an adult kython. The monster finder doesn't have all sources, but it has a lot of the big ones, so it does the job well enough. No reason to toy around with crossing the edition streams, and allowing in all that inclusion bias and general oddity.