colocasia can be grown as aquatic plants, not sure about alo, I would guess no.

if they are in pure gravel, you will have to provide water more often than is reasonably possible since the moisture retention is non-existent

I also overwinter EE's and cannas for my pond. I start them inside in soil and once they are rooted and sprouting I wash all the soil off and put them in baskets with rocks to weigh them down and hold them in the baskets and put them in the pond. No soil whatsoever. I have been using this method for years with great success. I found out the hard way that if they don't root and show some top growth before being immersed in the pond the bulb will rot in the water. I don't use soil for any of my pond plants...too messy etc.