Being that you want a tank full of them, I would go with a smaller species and as far as what is on your list I really like the looks of the flash pleco and could see a tank full of them looking really nice, not to mention you could attempt to breed them. and if it were me and I was setting up a "pleco tank" I think I would go with huge school of tetras or some small schooling fish just for some movement.
I could see the royal and flash plecos possibly being an issue for the reason that I believe they are more herbivorous where the others are more omnivorous. Also the royal can (eventually) grow to 18" and the gold nugget can grow over 12" too. I would wait for responses from others with more experience with mixing different species though.
Gold nuggets are hard to aclamate [sp]. I have a pleco only tank, and it is cool to sit in front of and watch. Also it is kinda like a wheres waldo game trying to find all of them. I built a under sand jet system for mine to create currents underwater, and the plecos love it.
As far as which ones to put in the tank is tricky. The clown plecos stay small under 3 in, brushy's are another good choice, so are rubber lips. the green phantom is a pretty popular pleco. You will need lots of caves and hiding spots as well as a selection of wood for them to chew on.
like others said do your research before you buy them and make sure they will work together. Some are more teritorial than others, also make sure you check about what they eat. Because you dont want to feed meat to non meat eaters.
there are just so many species of pleco's and most of them are really cool looking fish.
Ok ill still be doing my research then , it will prob's be a month or so until i do start getting some Pleco's if i do decide to go with a Pleco tank .
I have a few large peice of Wood for the tank , a large peice of PVC pipe that i can cut up for caves and have seen a few of them Slate caves on Ebay which say there are for Pleco's which look nice .
I do like the Royal Pleco alot but as someone said before that they are more a herbivorous Pleco they could be a problem with them i with the other Pleco .
I aslo like the Flash Pleco but they could cause a small problem too for the same reason !
I believe a herbivorous diet for your commons wont hurt them by any means. The flash pleco is a herbivore along with the royal. So if you like them I would say go for it and if space becomes an issue later on I would trade in the commons. The size difference when they are full grown might be a concern. 18" for a royal and around 5" for the flash. If someone else or yourself believes they will work until the royal gets much bigger than the flash, maybe you can look into the dull-eyed royal L-191, they apparently only grow around 10".