I would check with the your local
electrical code. If I remember correctly
here in Ontario Canada, a 15A circuit can
have a maximum of 8 fixtures attached to it
(I would have to double check that to be
sure though). Assuming the code allows you
to do simple math to calculate the maximum
number of fixtures for a 20A circuit, you
could only have 11 fixtures per circuit.
As far as putting 660W on a 600W dimmer, I
wouldn't do that. You might be able to get
away with it if it is a temporary setup and
each switch is in a separate electrical box
(not ganged together). You would have to
check with the manufacturer, but typically
you have to subtract 100W from the rating
for each additional dimmer that is in the
box (2 in a box, each would be rated at
500W, 4 in one box, each would be rated at
300W).