We had a pair of Ancistrus spawn in our 125 planted tank, and there were probably 40 or 50 in there at one time for quite a while - up to 2" long. There was no damage to the plants at all that I could see, either from the parents or offspring (we called them teenagers ).
I have one of these in a heavily planted 10 gallon tank. He is as hardy as hell and has managed to survive despite the fact that the tank is overstocked with fish, there is virtually no algae in the tank(perhaps he is alive because he has been keeping the tank free of algae) and he rarely gets fed. He has not harmed or uprooted any plant leaves. I read though that they can go to town cleaning algae covered sword plant leaves and damage them due to their rough handling of the leaves.
I have to add that the bushy nose plecos that I have had are good except for:
any large leaved plant such as the large amazons and java fern.
to wit, they absolutely kill these leaves. They scrape them down to the skeleton and the leaf is useless. Cut it back and they will regrow, but they get EVERY leaf on the plant, and you end up cutting it down to nothing as they look UGLY.
I like the fish, but they wreak havoc for me on these types of leaves.
Tried to do the same to my A. barteri, but they were too tough.
Leaves all else alone and does eat algae.....
That is MY experience, which apparently differs from all others LOL
I have only one BNP and he does this to EVERY large leaf, to the point where I can either have the fish or the plant, NOT YET BOTH.
I noticed that my Bristlenose in my 10 gallon hid a lot. There were times that I wanted to scour the tank and look for his dead carcass as I was sure that he was dead considering that the tank is overstocked. Then out of the corner of my eye, I woud occasionally notice his little tail flapping about as he was probably busy feeding off some dead plant matter or algae. Now, I see him coming out more and more even when the lights are on. I was beginning to think that maybe he was sick as this is not his normal behaviour. Then, I realized that he only came out to the spot where I place some microwaved zucchini. My guess is that there is probably little or no algae for him to feed off or dead plant material so his hunger is forcing him to come out in daylight where the zucchini is placed. He usually devours all of it within minutes, then disappears like the Loch Ness Monster.
I have not noticed any Bristlenose induced plant damage, and it has been many months that he has been in that tank, so I am convinced that he will not harm the plants as long as he is fed. Some say that BN Plecos can do a number on java ferns to the point that there are no java ferns left. I have two java ferns in the tank and just added a third. They are perfectly intact and the other two have been for over a month. Mind you though, I was fortunate that there was no algae on the java fern leaves and no algae formed on them to date, otherwise I am not sure what he would have done to them.