nathanh,
I think you do not need any special software for sizing a generator. We even make load forcast for rural electrification for small villages.This is for villages where there is no electricity! In the load forcast, we list up the number of households with sizes (single storey or two or three storey etc), list of lodges, hotels, restuarant, commercial buildings, Govt. buildings, small scale industries, big industries (more details of the equipment to be used including time of use and starting current etc), service sectors like water supply, battery recharging, religious shrines irrigation pumps etc.
I think in your case you need only to take into account of an industry. For this you can make a spreadsheet in Exel. List up all the loads, starting current, reactive power needed and time of use. Make estimate as accurate as possible. The final result will give you preliminary sizing. From the corporate get the data for the plant extension for some years. The third part is to get how much budget you have in order to make up mind to procure the size into single generator or multiple generator for financial outlay. This will give when you should purchase plant of which size.
I think this will give you fair idea. If again you have problem, other than national regulations, which may also become serious in some cases when you do not know the pertinent clauses, please send further querries.