I don't have a spare 5 gallon carboy laying around. Can I use a 7.5 gallon bucket that is used to ferment beer?
Yes you can.
Might as well make 7.5 gallons while you're at it.
I'm just saying...
not a problem. I've done it in a pinch, however I agree with Ron, just spend the extra $5 and make 7 gallons. I guarantee you'll drink it.
"Those who dance are considered insane by those who can't hear the music" --George Carlin