No problem using a whole packet of yeast in 2.5 gallons and no problem fermenting 2.5 gallons in each of your buckets.
As for the hops, they'll vary, sometimes considerably, from crop to crop, as well as where they're grown and even between fields in close proximity to each other. Your hops may be very different than what you'd get in a kit. At least with commercially grown hops, you'll know the AA%. That's why most homebrewers use homegrown hops for late additions and dry hopping when their affect on bitterness is minimal.