Yes, you can. With small number of disks that is the only practical configuration. Even C-Mode now allows it (indirectly via disk partitions, but it is still the same - disks are shared between root and user voumes).
Advantage - disks are shared between multiple workloads.
Disadvantage - disks are shared between multiple workloads.
Seriouosly, there is no black and white, espicially in IT. What is advantage to one, is disadvantage to other. Separate data aggregates are more flexible (you can move data to another filer) and reduce impact of workloads on each other. Multiple small aggregates waste space and reduce maximum performance for each workload. Pick your choice.
in cDOT you cannnot store data in the root aggregate however, as you are running in 7 mode your root aggregate contains vol0 whcih is where ONTAP system files live. It is fine to create volumes in the same aggregate vol0 lives in.
Personally i would want vol0 to be in an aggregate of its own and named accordingly (aggr0_root) which is sized approriatley, but thats just me!