Well honestly I think you should bring in an electrician to discuss your options as a tank should have at least one dedicated line especially one with such a high load. Even if you manage to get this working as is you're taking a significant risk. Once your tank is established it'll be a significant investment and blowing a breaker when you're not around could result in a total crash in a short period of time. MH ballasts draw a large amount of power during start up and if you have a brief power trip in the house all the ballasts would refire at once so staggering the times they turn on isn't a reliable solution unless you have a controller which will restagger after a power outage. Also as others stated heaters could kick in at different times and change things as well.
The other options I could suggest are:
- Downgrade the tank lights, chances are you could change the 4 250W halides to 2 SE 250W halides in a larger reflector like the lumenmax which will give better coverage. This is enough for a 180 gallon but you'll limit yourself a little with stock but could still keep most LPS, soft corals and even a few selective SPS in certain spots. Other lighting options like T5 (12 3' T5s is only 468W, even 16 is 624W) or LED may also be worth considering. Using lower K bulbs will increase your PAR as well if you're flexible on color.
- Invest in other equipment with lower power consumption
- Change the room lighting as already discussed, not sure on the exact bulbs you have but you can get CF in a GU10 base, they just aren't dimmable. Also there are LED lights that match a 50W halogen, they just aren't available locally and cost a lot more.
- Last option would be downgrade the tank to something you can manage with out blowing a circuit or your budget.