I would avoid clown plecos (usually panaque maccus) in the planted tank.
IME they are mostly herbovoirs and wood eaters so they would certainly enjoy nibbling on some plants. Over time they devour driftwood which is turned into wet sawdust in copious amounts.
If you do decide to try them it will be helpful to feed good amounts of vegetables. I usually feed zuccini which they love. To avoid having it floating around I usually impail it on one ot the little airlineholders with a suctioncup and stick it to the glass near the bottom. If you use raw zuccinin it will last a little longer then if you blanch it.
This may discourage the planteating.
They will also eat algae. IME the glass in their tank never needs cleaning.
IME, most herbovoir and omnivoir plecos will suck on plants if they don't recieve enough bulk vegetables and usually go for larger leaved plants such as sword plants.
Larger plecos can also uproot and rearrange the aquascape.
IMO the best plecos for the planted tank would be of hypancistrus(Queen Arabesque or Zebras for example) and perhaps some baryancistrus (gold nuggets for example - they are supposed to get large , up to 10" or more but I have yet to see one over 4")since they are mainly carnevoirs and usually stay smaller though I would keep an eye on any baryancistrus, at least in the beginning.
These will do well but enjoy more watermovement then is the norm in a planted tank. They won't do much for your algae though .
I have several plecos on my planted tanks and they don't mess with the plants at all, if you feed them algae wafers they will devour them and leave the plants alone, it's important to always drop the waffle in the same place so they get used to it and always look for it in the same place.
I have 1 clown pleco and he does fine and doesn't cause any trouble at all
I've always heard that the Clown Plec is a Peckoltia. I have one in a planted tank and he's never touched the plants and does a great job on the algae as well. Another bonus with them is that they are about the smallest Plec out there.
I've also had no problems with any of the Bristlenoses bothering plants unless they have very delicate leaves and they rasp a bit too hard.
I have kept at least a dozen different L-numbers in my planted aquariums without any incidents, and I believe that there is a lot of misleading information concerning their behavior.
Many of the places where the more common L-numbers are caught are rather without vegetation, and a great deal more plecos than previously thought demonstrate a tendency toward carnivorous feeding habits. I doubt plants are a significant element in most of their diets, and the dentitions of many of them support this. Without a doubt there are herbivorous Loricariids, but I don't think they're all that common. The standard LFS-available plecos seem to prefer meaty foods and algae (wafers) above plant matter. Well, at least IME.
I personally have had more problems with snails than any pleco. I think the main issue that you need to watch out for is their impatient romping around on the substrate, uprooting plants that you are trying to get to take. I'm still having trouble with my Ranalisma because they continue to knock it around.
A clown pleco should be fine, but it all depends on which one you get. I can think of a dozen different L-numbers I've seen listed as "Clown Pleco", and there are oodles of species in the family with similar/identical markings.
Mine come out when the food goes in, and after dark they like to scrape the algae off of the filter lift tube and the overflow ramp. It's kind of cool when they do that, I can hear their teeth scratching on the plastic.
I totally agree, a lot of plecos (loricardiae) are carnevoirs and don't bother plants, just as you mention - a lot of them come from fastmoving streams which are deficient of any plantlife. This includes the zebra (L-046,) Queen arabesque (L-260) as well as some slightly larger ones : redfinned leopard (L-114) spiny monster (L-096) to name just a few.
These , like most fish, well benefit from a planted tank due to the plants oxygenating and watercleaning properties. The only problem may be that they prefer fastermoving water.