Nice article! Just one tweak:
"If one page in an extent is owned by TableA or IndexB, all eight pages will be owned by that same object. An exception is made for very small tables and indexes, ones that would not fill and entire extent."
Close, but not exactly. The first page of a newly created table is put into a mixed extent. When we first create a table, we don't know whether it's going to be a "very small table" - every table starts out very small. After the table has 8 pages, then future allocations are put into uniform extents, but the original pages aren't moved to uniform extents automatically. You can move them yourself by rebuilding the clustered index, though.
You can read more about it here:
Also, just to nit-pick, heh - there's a typo. It should say "would not fill AN entire extent."