My personal experience, The best substrate is zero substrate. Make it a bare bottom tank. Why? Because they are some heavy poopers, I have 9-10inch common place and a 4inch bristle nose place. I'm constantly having to clean the bottom of my 75g tank because poop just accumulates. Also after a month my fluval 406 is just filled with poop. If I squeeze any and all of the sponges it feels like I'm squeezing into some thick nasty mud. But I know that ain't mud.
Also places generally get pretty big and are bottom feeders. Bottom feeds, scavengers generally will eat anything on the bottom including dead fish. And they poop alot! So good filtration is a must. Also plecos in the wild swim over long large surfaces. They need huge rivers and lakes. Not the tiny tanks we put them in. They kick up a lot! Of dirt and sand. You cant have some fancy big planted scapes with most plecos. Because they will just destroy it. So with all if that being said. I wouldnt use any substrate. If your going to get a timiki/bristle node I think their max growth is 4-6inches. That's more doable. Then I'd use sand or any of the plant soil substrates. Gravel would be just fine too.