I learnt from my Tradesman Plasterer father many years ago that Salt and Milk were used to alter the set times for Plaster. I had forgotten which did what. While visiting him today I remembered to ask him, so here goes.
Salt will speed up the set time, the more salt the faster it sets (not much salt is needed).
Milk will slow the set time, the more you use the slower it sets (again not much is needed). (this was in the days before fancy milk varieties so just plain milk unless you want to experiment with other types).
The plaster i use is normal builders plaster available in Australia in 2kg, 6kg, 12kg and 40kg bags.
This type of plaster is much cheaper than Hobby shop plaster, by adding a little Salt it suit our purpose well.
Try a few small batches to determine your requirements. Don't throw these away use them on your layouts.
If casting tunnel portals or rockery. i'd suggest a fast set, if you want to carve cliff face etc a medium fast set would be better doing a small section at a time.