I have just lost my 4 due to White spot, they were super little fish.
My tank is 30 Gallons ,and they used all the space because they zoomed around so much. I suppose 20 Gallons would be the smallest size i would personally keep them in. Two of mine were fully grown at 2 inches.
I wouldn't say they shoal as such, but much of their day is spent playing. mine loved their Slate cave ,and quite often they would rearrange the Gravel inside it .They are smashing little fish and i thoroughly recommend them .
B.Sidthimunki normally max out at around 2", I have seen bigger specimens including one that I owned but generally 2" is about as big as they grow.
Tank size depends on how many you are going to keep and how happy you want them, I have 10 at the moment in a 33 gallon and soon they will be moving into a 45 gallon along with some B. Nigrolineata. I personally wouldn't put them in anything under 30 gallon as they love to swim and are easily the most active fish in the tank, as for shoaling, mine shoal up pretty much all the time its very rare that I see on of them on their own.
Here are a couple of pics of mine shoaling and sitting on bogwood together: