I would like to include some marsh marigolds in my pond to help decrease algae. I would like to plant them in small pebbles rather than soil. Will they grow under these conditions?
I do not believe you would have success planting in pebbles as a planting medium.
Pond plants may be a better option for you.
Here are a few links with more information.
https://www.gardeningknowhow.com/ornamental/flowers/marsh-marigold/growing-marsh-marigold-plants.htm
https://www.gardeningknowhow.com/ornamental/water-plants/wgen/oxygenating-pond-plants.htm