Howdy,

sorry. I'm late.
Plants and Piranhas aren't all that hard to combine. Get your piranhas young, so that they aren't big enough to uproot plants. This gives plants time to anchor. Get some good substrate ferts into your set-up and good lighting. As for algae, keep your nitrates in check with water changes. I noticed that high flow can also cause algae (bearded algae). Reduce or disperse flow.
When large enough, you can add all sorts of animals to your planted piranha tank to help with maintenence. I have trumpet snails, cherry shrimp and Endlers with mine. I am sure otocinclus would work also once your Ps reach a foot in lenght.
All in all, plants actually help with water quality in tanks of messy fish like piranhas.
HarleyK