un bocal à poissons - (a fish tank is 'un aquarium')
yes they can but they tend to live way longer in a tank
If you mean a bowl like a glass bowl the size of your head that you keep as a centerpiece, no. Well, you can, maybe, while they're babies, if you have a filter, aerator, and don't mind your fish...
I would say that about 10-15 guppies per gallon.
It is boring to live in a small fishbowl and your fish will die quickly. It will be better off in a tank