BN plecos do eat algae, one of the best algae eaters you can get usually. If you feed yours algae wafers or other food regularly, you have probably trained yours into thinking they don't need to bother with the algae, as they will be fed wafers. In tanks where I grow a decent supply of algae, I don't feed my bristlenoses anything else, except the occasional cucumber treat. That keeps them busy on the algae. The Oscar aggression might be an issue if yours are aggressive, each fish is different, though generally they are tolerant of tankmates.
The flagtail is do-able in your tank, I just don't think it will do as great of a job on the algae as you might hope.