streams are not for ‘real time’, their purpose is to allow large amounts of data to be buffered up so you can hide the slowness of the media, netstreams and cdroms for example.
If you want low latency access directly to the mix stream, add a dsp callback instead. you can either use addDSP to add it to a channelgroup, or even have it play on a channel (and get the features of a channel like being able to set the frequency of it) using System::playDSP
if FMOD wants 5ms of data in this case you just buffer your own reads somewhere else and feed from that buffer in the dsp callback.