I don't understand why people are so concerned with head space in the fermenter. I have put 5 gallons in a 15 gallon plastic barrel to ferment and that beer recently got sent on for national COC judging. I think you will be fine.
yeah, the only thing more headspace is going to do is prevent a major blowoff mess. nothing bad can happen from having extra headspace in your primary fermenter. it will fill with krausen and CO2 and your beer won't ever tell anyone that it was in a bigger fermenter than necessary.
I agree, the co2 creates a buffer space anyway, and if you close off the fermenter, and use a proper fermentation blowoff tube or lock, it won't have any issues at all.
Fermenting/Kegged/Bottled NONE :( I moved to the NW and haven't had time to setup my brew rig since! (but hey, I'm in the Pacific NW so there's so much awesome beer I don't need to brew it as much hah!