Yeah, definitely don't fill it all the way up with water. They will expand a *LOT* and you might come back to find water or crystals pouring out all over the place. I always mix up about 1/2 a gallon worth of crystals at a time. I put in way more water than is needed and let them sit out overnight at room temperate, then drain the excess water off the next day. This helps to ensure maximum
hydration of the crystals. Once that is done, then I move them into whatever storage container I choose to use at the time and place them in the fridge. You can keep them out at room temperature, just make sure there is an airtight lid on the container to prevent evaporation. I just happen to have more space in my fridge than anywhere else.
If they should ever dehydrate, you can always rehydrate them again with no problems as well. Have fun and I might suggest that you do some research on roaches versus the crickets. You will thank me one day.