I always just build fortifications as my anti-magma creatures screen. Usually about three, and a decent length tunnel to carry it all away from the magma pipe. Never have any problems after that 
Ditto.
I also build my magma forges, smelters, etc, where the block part is placed directly over the hole. That way even if they squeeze through the fortification (I've seen it happen once, I think it's a glitch), they can't get through the shop's magma hole and none of my dwarves can fall in.
Fire snakes will always be a problem unless you transport the magma out of the biome you get it from since they "teleport." Never build directly over or next to the pipe if you use wood (such as bins for metal bars).