I've done outdoors in the clay soil you mention.
It is very difficult to get enough *stuff* into the soil to keep it loose and fluffy.
You'll be much better off with a few sacks of compost than you will by using potting soil.
For best results you'll need at least 3 foot deep holes, and it's better if you can manage 4 foot...and yes, I do know how tough it is to dig that deep in rock hard dirt, it's a real ball buster.
Just remember that a plant normally has about the same size of roots under ground as it does of foilage above ground so don't limit it by too small of a area to grow it's roots into.
If you prep the soil well enough you shouldn't need to give it any fertilizers at all, the soil should easily be able to provide everything it needs for good growth.
Don't believe that fish emulsion bottle that says you cannot overfeed, you most certainly can overfeed it enough to fry it to death.
What I would do it go to Home Depot, Lowes, or Wal Mart, and get 1 large (2 or 3 cu foot) bag of "Supersoil" brand potting soil (it's not very good potting soil, BUT it is GREAT compost) and about a 1 cu foot sack of playground sand per hole you plan to dig.
Each hole will support 1 or 2 plants, 1 being best.
Dig a hole about 3 foot wide by 4 foot deep making a pile to the side of everything you dig out of it.
Line the hole with bisqueen or a sturdy plastic drop cloth, use 2-3 layers.
Mix the soil you took out of the hole with the supersoil, the playground sand, and about 3 cups each of blood meal and bone meal (available at the same type of places. Add about 2 cups of dolomite lime (agricultural lime) for a PH stabilizer and magnesium suppliment.
Mix well with your shovel and toss back into the hole....do not pack it down.
Water it very well and cover with another sheet of plastic for at least a week....secure the plastic down with some rocks or the like....the soil needs to cook and sweat a bit.
Take the plastic cover off the hole and plant away.
You won't have any trouble with the heat or the sun....as long as you keep the soil semi moist.
I have a feeling La Nina would do a bit better than the Shit as I know it's probably going to see some 110-115 degree days.
Post pics