I've had good hatches from eggs over 21 days old. A lot of it is down to correct and careful storage, as well as the age, health and nutrition of the parent birds. Anyway, as long as you've got room for them all in your new bator, what have you got to lose by setting them anyway and seeing how they get on? I mean, they won't all automatically go rotten if they're too old to develop - I've been incubating old and dodgy eggs for ages now and I've never had one go rotten. Most likely the worst that would happen would be that you'd get more non-developers than you'd normally get incubating fresh eggs.
I'd say, keep ALL of the eggs. Write the date on each egg as you collect it, then when you're ready to set them, if they won't all fit in your bator, then put them in in reverse order, i.e. the freshest first and then work backwards, so that the ones being left out are the oldest ones. Does that make sense?
What kind of bator have you ordered anyway? If it's one of those styrofoam ones that takes 40 eggs, you'll be able to set all of yours and still have room to spare!