a small as u can... but best would be about 2"
probably 8-9... they are still a little more peaceful if started as a group from small... and seem to work together to rip apart things a little better. then when put together as bigger fish.
The size is really up to you, but the small you get them at the cheaper they will be. Hence, you could start off with more. I would say that you could plan to end with about 7. So, I would start with at least 10. If you are lucky, and good at what you do then you can always sell off the excess. It is much easier to sell off a few than to try to add new ones to the tank.