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.