I am a big fan of extra space in a tank, I would say yes they would be OK, filtration will be your biggest worry, and if you happen to get one fish that is especially wicked or territorial, you will have to remove/replace it. A 125g is a pretty good size tank, and there should be adequate swimming room. Don't know how long the giant Danios will make it, they are fast, but they'll be in danger if they ever quit moving.