I have a 13g imperial keg for my HLT, serves me just fine for 10g batches. I need to top it off during the mash to be sure I have enough water to sparge, but I have an hour to remember to do that and get it to temp, so no problems.
To do a 10g batch, I usually start with 13.5g of wort in the kettle, boil off maybe a gallon and a half to two gallons, and lose about .5g to trub and hops. My goal being to get 5.5 in each carboy.
Since you'll need to start high and boil down, I don't think the smaller kegs would work too well as a boil kettle.
Edit: I know some folks on here who boil in smaller kettles use foam control products that could let you fill almost to the rim, I've never used one so ymmv