how bigs the fan? and im pretty sure most houses have a 10 amp circuit it would be hard to reach that point with just one light and a fan or two. but i would do your reaserch im not the smartest lol
it should say on the back of the ballast but most 600 watts should be right around 6 amps and a normal house fuse is at 15 amps (i think ) so you should be very safe to add a few fans on top of your light if it is in deed a 15 amp fuse. most small fans are 1 amp at most.
a simple calc is to take the watts and divide by 110 and you should have a close estimate of the amps
My 600watt Digi runs 5amps and my 1000w runs 9.5amps most breakers run 15 amps but remember you only want to use 80% or your allowed amps so if your allowed 15 then you only use 12amps......
Also my parents house has 15amp limit per breaker but my apartment has a 20amp limit per breaker so there not all the same....... if its a 600w then put it all on same breaker but if you running a 1000w you might wanna split some of the things up onto two breakers...... In my apartment i run 2 1000ws a 8" and 4" fan and 4 smaller circulation fans......so i must use two breakers to run this i put a 1000w on each breaker then split the two fans up on diff breakers and then i put the 4 small fans on the same breaker as the 4" fan since my 8" fan usess almost two amps...... I have to run an extention cord to my grow room because my grow room only has outlets run to the one breaker to i need a cord to run out the room to another set of outlets going to the other breaker........ hope this helps.....later