The maximum wattage for a 15amp circuit at 120 volts is 1800 watts. I like staying within 80% of that even for non-continuous loads such as lights. So, you probably do not want to go over 1440 watts especially with a cieling fan on the circuit. To calculate your total watts, you will have to consider the manufacturers rating for each fixture and not just what wattage bulb you happen to have in it at the time, someone else might use a higher wattage bulb. If the fixture is rated for a 100 watt bulb; that is the figure you will have to use for your calculation of adding up all the bulb wattages. The box will need to be changed to one that is rated for hanging a cieling fan. Hope that helps.
** I'm not a professional - only a DIYr **