Sure you can instantiate as many SQM instances as you have interfaces (it does not make sense to instantiate more than one per interface, and older sqm-scripts will not behave well with multiple instances on the same interface).
But note these different instances are fully independent, so that:
a) the WLAN shaper should be set lower than the WAN shaper to have any effect on data saving
b) the WLAN shaper's packets will still compete with your other packets at the WAN shaper, so there is no guarantee that the streaming devices will get a dedicated bandwidth, effectively you can introduce a hard upper limit but no strong guarantees beyond that.
c) Having more shaper instances will, in the case of actual traffic, cause more load on the router's CPU and might start to run into issues with available CPU cycles.
From my reading of your post, these issues (with the potential issue of c) ) would not matter to your use case...