As long as there is an opening in the ice for gas exchange, any hardy temperate species of fish should survive. What kinds of fish do you have? What is the air temperature? How big is the pond? It's okay to gently break the ice in or around the waterfall. I use the end of my pond net for light ice and a sledge hammer for thick ice. Others on this forum use a metal pole. It's important to not bang on the pond ice as the shock waves can harm the fish. If you're working on the waterfall and never hit too hard or too long, it should be fine to break some ice. On the pond itself, I think of it this way. In the wild, if an animal was walking on the ice and fell through, a certain amount of noise would be made. As long as you're not doing more than that, it shouldn't bother the fish much. So, I'll use my foot to break ice on the pond itself (while my other foot is on solid ground) but never bang on it.
My pond winterizing page is at
http://www.fishpondinfo.com/winter.htmGood luck with your pond!