Running and gunning with 5 guns gives you a way better chance and shooting others off the break and I have found that when everyone is looking down field it is easier to see lanes coming in and easier to doge then then just sprinting off the break.
I guess for speedball you would like to have a quite descent fire rate. A long, double finger trigger seems a logic choice (except if you can get really fast with only one finger haha). Maybe for a beginning, one could start with a GoG EnvY or the upper one (can't remember the name). They come at a really descent price. As soon as you start going into electronic guns (which should allow a really fast rate of fire), the complexity and price are getting bigger.
I might be wrong, I only have experience with the GoG Enmey which is more basic and casual I guess.