Wayne et al. have really "hit the nail on the head" so to speak: some of the things we seek to achieve in refugiums are difficult if possible to achieve to the same/highest level(s) at the same time.
Although we can enjoy numerous and simultaneous benefits from most any type of system - high/low flow, coarse/fine substrates, deep or shallow beds, lit/unlit, etc. - we really must focus on some primary goals and accept the compromises on other aspects.
You can imagine some really neat refugium styles/applications that are truly incompatible with each other: settling chamber or turf algae system? [slow versus high flow]... amphipod factory or, hmmm... spaghetti worm fetish? [coarse versus fine substrates]... Gracilaria farm or cryptic refugium [high versus low light].
Again... need to worry about a strict "rule" for flow
No such rule exists.
In the case of Chaeto and Gracilaria, moderate to high flow and perhaps tumbling of the matter will not harm but likely help the colony. No more, no less
kindly,
Anthony