Assuming that what you have is a typical half-barrel keg (which would be 15.5 gallons), I would recommend using the keg as the brew kettle (a 'keggle'), and the 7 gallon boiler as the HLT. This would give you ample capacity for boiling the wort in. If you were boiling down a ten gallon batch (or a truly dense five gallons - some Scotch wee heavy, anyone?), this would give you a maximum of 14 gallons brewing water (7 gallons for the mash and another 7 for sparge), which assuming around 2.5 gallons grain absorption, would leave 11.5 gallons for boiling. What kind of tier or brew-sculpture setup are you putting together, and what burners are you using? Just curious. With something this size, you really want a
three-tier gravity fed system, if you can arrange it. There are some commercial ones, like the
Blichmann modular system or the ones at
BrewStands.com, but those get pretty expensive.