when i quick-inspect the sources of racer, i see some requires related to redis. may be racer use it already under the hood, but i'm not sure about it. but see here
http://derbyjs.com/#persistence . there is a racer-mongo adapter out there, dunno about redis.
you not specify which model use which store. you create a model from the store, so the model is bound to the store where it comes from.
scoped models are just models with reduced path-scope. so when you define a path "collection.obj1.property1", you can get a scoped model with model.get "collection.obj1" and query paths like scoped.get "property1". thats it the paths will be not resolved globally, but by prefixing the path to the scoped model.
you should really read the docs and try a bit with simple apps
:)