Not stupid thinking, just wishful thinking.
The pleco may not be herbivorous, Even if he isn't then his rummaging around will do a lot of damage. I doubt there's any way that you can get plant growth to stay ahead of the pacu. It isn't just a matter of appearance.
Bolbitis are ferns with tough leaves that don't seem like they would be very interesting to an herbivor. It needs to be attached to wood and the pleco may not allow that to happen.
A lot of the other fish that we usually consider appropriate for a planted tank would probably just be food for the arowana.