I wouldn't plant anything on the pipes. How about covering the hole with a large pot and planting in that? And if your nursery told you Cordyline is a palm I'd change nurseries.

I plant over pipes all the time.. but I would not plant a large palm (not deep enough hole)…. but a lot of smaller ones would be OK (ones that never get too large). Palm roots are not terribly damaging to surrounding structures, but those pipes are really shallow. Other option is cut off the water, reroute the pipes to go along the very edges of the hole, and then you can plant something larger.

A Cordyline palm? You need to switch nurseries, brother.

I feel your pain. Happens to the best of us. I wouldn't plant over that anything taller then a clump of ground Aloe's.

What might work? Take out a 2-3' of the section under your left foot (or whichever direction faces the pool) plant your trio of King or Alex palms there. With a ground cover under them? You wouldn't even notice what shape the opening is.

Of course if you enlarge the opening clean and even,then maybe rock or brick mortared around that- I know you know what I'm getting at.

And stick to small or moderate palms,never put a CIDP or Pindo or Washy there.

What was the decision? Just curious.

I wanted to keep the tropical look around the pool so I ended up planting Pygmy palm ,according to this link ,they have shallow roots

http://homeguides.sfgate.com/palm-trees-shallow-noninvasive-roots-37598.html