Outside of a goldfish which would eventually grow too big to stay in a 10G and snails, freshwater critters need heaters. It's a shame not to get a betta for such a beautiful tank, and the filter you chose is perfect for them.
Depending on where you live you could probably get away with a very low wattage heater. A 50 watt heater can easily heat ten gallons to the temperature you need, and a heater that small really presents no danger at all. Any heater you get now has a safety shutoff built in, but even if that should fail fire and/or exploding heaters is not really a risk.
As far as water conditions, if it's filtered and you add a water conditioner/dechlorinator it should be fine for a betta. They're not fussy about pH and such.
Appreciate your help and reply. I'll stop hijacking this thread with my questions now. :) Thanks again everyone and keep posting those beautiful tanks!