As tough as onions are, I'm not sure they'd last a month or two in a fridge since the air dries them out pretty bad in there. If you're trying to stagger the plantings so that they mature at different times, you're pretty much out of luck since onions have that photoperiod thing going and they start bulbing when the day length gets to a certain point depending on the type of onion grown.

For green onions, what I do is plant the starts at 2" spacing in the ground and let them grow. Then, as I need green onions, I just pick every other one and the spacing is pretty close to right for bulbing onions. When I could only get to the garden on the weekends, I would also plant any extra in a 36" window box at 1" spacing and just harvest them for scallions as needed until they were all gone. Worked well and the walk to the apartment balcony made it really easy.

I must be jaded but 300 onions doesn't seem like a lot to plant all at once. If you're growing any for storage, a hundred or so of good storage onions may not get you through the winter and harvesting the rest for scallions will really depend on how much you use onions. Red Burgermeister (sp?) makes a very good table onion though for me the torpedo shape of Red Tropea is good for salads but not so good for things like hamburgers.

Check out Dixondale Farms for onion growing instructions ... their plants perform much better than sets, you get a great selection at a very good price and they ship when you want them.