Hi Nudenerd
I am afraid that adding a betta to that tank will be a mistake. They tend to go for anything that looks remotely like themselves. This might happen after a day or so when you think everything is going fine. I have tried mine with various fish and he attacks gouramis, hatchet fish, and black widow tetras. I have never kept guppies but if they have the trailing fins they will definately be a target. Mine is fine with corys I think this is because they hang out on the bottom of the tank. bettas do vary they have individual characters and you might have a more peaceful one but i wouldn't risk it. I ended up having to get an extra tank. Also some people say that bettas don't like big tanks as they get stressed having so much territory to patrol.
DD
