With regard to grafting C. edulis on C. tetrameria or visa versa, there is no incompatibility.  I have 15 year old C. tetrameria grafted on C. edulis and the tree produces prolifically normal sized fruit.  The tree is about half the size of C. tetrameria on its own roots.  C. tetrameria is not a huge tree like C. edulis (about size of a peach tree).

However, grafting C. edulis on C. tetrameria (white sapote on yellow or wooly leaved sapote) leads to extreme dwarfing.  I have Pike and Vernon grafted on C. tetrameria rootstock and 15 year old trees are no bigger than 3 ft tall .  One is still in 5 gallon pot, other in the ground.  Both are same size.  Production is limited by size of tree, but normal sized fruit.  Each produces 3-5 fruits.

Crfg years ago did a taste test of the yellow sapote (wooly leaved sapote, C. tetrameria) and labeled two as worth propagation.  These were identified as #6 and #7.  Taste is different than C. edulis, color is yellowish inside, seeds are much smaller.  Indeed the underside of the leaves are very fuzzy (wooly), C. edulis is smooth, except Suebelle has a slight fuzz as possible indication of mixed heritage.

Here in central coast of CA one tree (Pike/Vernon) has been dropping fruits for a month.  The fruits cover the ground and there are more to come.  The tree is a giant and has been in the ground for over 40 years.  Flowering is beginning on many of my other white sapote trees but they still hold fruit.

Try this database:

http://www.avocadosource.com/AvocadoVarieties/QueryDB.asp

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Fruit falls from the shrub (tree).  It is good on the ground for less than a week. There are selected varieties mu ch larger than a large hen's egg (Mammoth, Triumph among others)  Hardy to about 15 degrees F.  Ours have taken 19 degrees with no damage whatsoever.  Nothing bothers it, except birds eat the petals of the flower not inhibiting fruiting.

Tejocote punch is a traditional seasonal drink in Mexico.  I agree on the description of the fruit's flavor, but it is popularly utilized in other forms.  Google gives several uses for the fruit. 

Glad to note that, Peter.  mammee americana really succumbs to cool winters in my greenhouse, but still have one in 15 gallons.  I do fruit the green sapote in the GH, so maybe this fruit might be possible.

Actually they are probably more than 20 years old.  The 3 1/2 ft plant in the ground produces 6-7 normal sized fruit.  The one in the pot produces 4-5.  My yellow sapotes (c. tetrameria on C. edulis roots) produce 40-50 fruits on a 7 X 7 ft tree.  Now the Vernon/Pike white sapote (C. edulis) on white sapote rootstock is over 40 years old and so loaded with fruits that the undergrowth is loaded with squishy fruit.  A dwarfed white sapote tree would enable folks with small yards to enjoy this tree/fruit.

Something which I have not tried yet is using an interstock of various lengths between the rootstock and the preferred scion. This is done with apples.  The interstock could be any related variety (like tetrameria or pringlei) and enough time to see what works.  My C. edulis trees on C. edulis roots are giants and leave massive numbers of fruits on the ground for the local varmits and chickens.  A dwarfed tree could be an asset for smaller yards.

Here on the central coast, several are excellent (besides Hass).  Jan Boyce, Stewart, Pinkerton, Edranol, and Hellen (2 L's).  Looks like a good year this year after last year's rains.  Hopefully more to come this year.

William..Creamhart has a story attached to it.  It was registered by Orton Englehart and a good friend, John Moore, got scionwood and grafted a tree in N. Baja at an orphanage that he worked with.  He had a "demonstration garden" with many CRFG plantings.  John has passed away. Bob Holzinger went back to Orton's old neighborhood, talked to current owner and neighbors and was not able to locate Creamhart.  I spend winters in La Paz, Baja, Mexico and stopped twice at the orphanage driving down talking to people and walking the grounds.  Again, their emphasis is macadamias (another story) and the avocados are pretty much unmarked.  So no luck there in Baja.  Harvey (CRFG) found someone who said they have a seedling of Creamhart and provided me with a scion since I have a small collection of avocados.  The scion is grafted on a multigrafted tree and has produced excellent fruit.  I have labelled it (as Harvey identified it) as Creamhart F1.  This year I have grafted it on rootstock to make a singular Creamhart F1 tree.  Is it or is it not?  I don't know, many have looked for this elusive tree, with no success.  I met Orton with John and have a great deal of respect for his inventions (Google Orton Englehart) and his plant registrations with CRFG and will grow the avocado out since it is an excellent fruit.  The identification is Creamhart F1 in honor of Orton whether it is actually the original tree or not.  Perhaps a forum member reading this can add to this saga and point us in another direction.  The only help I had from Julie was she thought the tree was in Baja, Mexico.  It apparently was, but lost in time. 

More adventures in tracking down the many Edenvale Feijoas out there.

Allspice is a good one.  Try Oscar at Fruitlovers for seeds.