1. Depends on the tank size. Under 10g, only 1 female. 20g and up, you could probably do a group (not 2 unless the tank is big enough for each to have her own territory (ie, 29g+).
2. Most anything can be with female bettas, aside from males. This may vary with the individual betta, but unless you have a really nasty girl they don't care about the tankmates that much.
3. I doubt there's a max size, but the bigger the tank, the more females you would need so that none get an actual territory. Of course, in a nice big tank you would have more room for territories, so you could instead have a small number of females with territories.
4. Normal behavior is some flaring at first (they flare like males). Mine have always set up little territories in floating plants or in clumps of plants (the stronger one claims the floating territory and if there's another female she gets a clump of plants near the bottom). They aren't real active swimmers but they do go around the tank occasionally... they prefer to stay in their territory/defend their plant clump. With multiple females, you'd want to watch to make sure that all are getting access to the surface when they want, and that they are all eating. You don't want everyone ganging up on a weak female... this is why you'd want everyone to have her own territory OR enough females that there are too many for any of them to have territories (same as overstocking a mbuna cichlid tank).