A custom, non embedded (does not support embedding) DB cartridge can be installed to a non-scaled app, but does not deploy a new gear (or an embedded gear).
rhc create-app foo php-5.3
rhc add-cartridge "http://cartreflect-claytondev.rhcloud.com/reflect?github=smarterclayton/openshift-redis-cart&commit=no_embedded"
Broker reports success, but the redis cart is colocated with PHP (despite not having 'embedded'). SSH to main gear and there is no "redis" dir, which means that the install logic failed.
The cartridge cannot be removed because of bug 969724.
Should probably not block sprint exit, workaround is to add the 'embedded' tag. Long term we want to disable embedded, so we want folks to not have to set it on their carts to function properly.
The keyword 'embedded' holds no significance to the broker. If it is a non-scalable app, the broker forces all cartridges to be located on one gear.