If you're married to those fermenters, you are married to their dimensions.
I have a 7cu', and can fit 2 of the thinner 6.5G carboys in the 'deeper' section, and an ale pail on the hump, but I just use a blowoff tube instead of an airlock to deal with the height issue.
Another thing to keep in mind, if you are primarily making ales, is that active exothermic fermentation is only really going on for the first few days. So for example, if you brewed one weekend and wanted to brew the next, remove the first fermenter from the ferment chest, and put it in a place that keeps around 68-72 to make room for the second weekend's beer.
Finally, remember you are monitoring FERMENTATION temperature, not ambient temperature of the freezer/chamber. So you would only really need to control the temp of 3 different fermenters if they were the same beer at the same stage of fermentation.
Good luck with your selection!