This is really a no win situation. If your regular people are really good, you don't want to take a chance on losing them, because they aren't getting the hours they want. But, on the other hand, if the 2 fill ins are really good, you don't want to lose them either. BUT, I think it would be more important to keep your regular full time people happy, because you know for sure they are good and can be counted on. The fill-ins are basically unknown quantities. They may be really good when they fill in, but they might not be when they have more responsibilites and hours. Maybe you could tell them, they will be the first ones considered when you have to add more full time people and leave everything as it is for now.
I realize that this has probably been no help at all. Bottom line, though, you have to do what best for your business. If you have hours to add to your payroll, go for it, but if adding people would cut the hours of people you know are good, you can't take the chance of losing them.