Make sure any gravel you use is well tumbled pea pebble, or smooth round river rocks, with no flat/sharp sides. I have found the pea pebbles sold by the bag at Tractor Supply to be pretty decently tumbled and smooth. One thing to note: gravel is a string algae magnet and once it grows in it, it's forever. I'd skip the gravel completely. I only use it in my upper "bog" teir and regret even that much a lot of the time.
Haven't had issues with algae, pond was rebuilt back in March. Thought I'd get them, but not at all for some reason. Only some on rocks, but nothing like string algae. That will come in the winter I think due to all the leaves falling in there and the amount of sunlight it gets then. During the growing season, the trees cover the pond and algae can't really thrive as well as plants accustomed to shade, or so it seems. Even during the summer now, where there's 40c on some days this month, no algae, and I feed the fish a lot these past few days. But I don't know, I found last year that only in winter I had an issue with algae, and my guess is without sun algae just can't propagate as well.
I don't know, I've read with time bare liner becomes not so bare and gets covered over time so it does look natural. But I'd certainly have to wait few years for all that to happen. Most of the gravel is round, but PVC liners are sensitive so who can say. I had walked in there a couple of times this past week moving rocks, hopefully that's not the case.
