Multiple Sectors: The level of sector capital required goes something like (but not exactly) the square of its radius in jumps. So that means that it's more efficient to have a bunch of little sectors than one big one.
Hmmmm just realized that the "more efficient" statement actually depends on how the number of occupied systems grows with sector radius, which depends on the way jump points are generated, and average occupancy. Anyway, my experience has been that huge sectors are prohibitively expensive.
Multiple Fleets: The fleet bonuses only apply when the a TG is in the same system as its TF command group. Since the command group can't be in multiple systems at the same time, that leads to ....
John