the key to starting any tank is patience! Most people try to stock their tank right off the bat and usually they end up with those spikes of ammonia and nitrite because the bacteria in the tank can't grow fast enough to handle the load. If you are deciding between 10 or 20...go 20. Once you have your 10 set up for awhile you'll wish you went bigger.
Once you set up the new tank, let it sit for a few days to let any saturated gasses dissipate out. Then start with a few strong fish. My personal reccommendation is a type of danio since it is a minnow fish that can withstand quite a bit. If you want something a little more colorful to look at, go with 2-3 gouramis (a relative to the betta). These fish are not extremely dirty and they usually can withstand the hardships of the nitrogen cycle.
If you leave the two fish in there and regularly use a bacterial supplement (Cycle, Stability or BioSpira are best) then you should have a smooth transition through the nitrogen cycle within 4-5 weeks. If you do water changes during that period it takes a little longer but with very few, hardy fish you shouldnt worry about that. Do not add fish till the cycle is done.
Once the cylce is complete, only add 2-3 fish at a time and give it 1-2 weeks before you add more. When you do water changes, only change about 25% and add a bacterial supplement when you do. Also get a good filter that holds bacteria well without you disturbing it. The Penguin Biowheels are awesome for small tanks like that. Just remember that the key thing is to be patient when starting up and stocking a tank. The majority of the problems my customers ever have is usually because they rushed things.