I would take the rocks out of both tanks. They look awesome, but the added space will be better for these skittish fish and safer, especially in the smaller tank even with smaller fish.
As for tank mates, they need to be fairly aggressive and large. But, remember, these fish are pack/schooling fish so anything is fair game when its feeding time. Best to get the smallest/youngest you can and feed them regularly and consistently. Pleco should be ok if it can hide...however, if Piranha want to eat it they will...and you wont be able to stop them.