For a Orchid mantis (Hymenopus coronatus) the container should 12"x8"x8" for a large female adult - as it as good to have them in a adult sized container even as nymphs to prevent mis-molts due to size. As found
at this link for the species, here are the details about containers...
Also you can read the
caresheet here on to care for the mantid species. The species is conciderd a intermediate care species and not recommend for beginners.
Regarding fruit fly (FF) cultures all have mold/decomposition to some extent as can be understood by the smell and the FF need it to lay eggs. In a perfect setup the flies and larvae keep turning the culture medium by moving on it, keeping the mold from growing out of control (or making it even noticeable). You can buy commercially made FF medium that has molt inhibitors in it if you prefer rather than make your own.
Bacterial should not be a issue from FF cultures, I haven't heard of the issue before. The only real issue will be mites, but that happens in all FF cultures eventually (they look like dust) when the culture is dying off. Generally mites are of little concern and die off when you throw away or clean out the old FF culture containers. They exist everywhere but only become a concern in FF cultures when the are out pacing the FF population.
Either way the FF themselves eat the medium and digest it, and that should not affect any mantid nymphs. If it was a real issue then FF cultures would kill off all nymphs and the mantid hobby wouldn't exist.