Secondary is optional, but why would a high temp pitch have anything to do with it?
As mentioned above, you can always add water to get to the volume and gravity you want, most extract brewers do anyway (partial volume boil). That is awesome you have temperature controlled fermentation, that makes a huge difference and most beginners don't have that. I would suggest setting your temp controller to 65-66 though, as the temp of the fermenting beer inside is likely a couple degrees higher than the fermometer shows. Mid sixties vs high sixties tends to make cleaner beers for most ale yeasts, which you want with a pale ale. Also realize your recipe will be a bit unbalanced now with that much of an increase in gravity, but not bitterness. So it will be a bit stronger in alcohol and malt, and less so in bitterness, either way you are going to love it, because you made it! Homebrewing is awesome because you get to drink your lessons, and learn from them at the same time
And yeah, just buy more buckets dude! $15 a pop, get two and then don't worry about it. The beer really only needs to stay temp controlled for the first 3-5 days, then you can pull it out to room temp and put a new fermenting batch in. And repeat!