And if a hybrid of a native, does that automatically make it a native? I can't find it on any native plant list for Texas, just the regular yaupon holly, the tall version. Can anyone point me to a site with authority to say yes or no?
The dwarf Yaupon Holly is a selection of the native species, Ilex vomitoria and is usually identified by a cultivar name, such as 'Nana' (female), 'Stoke's Dwarf' or 'Shillings' (male). It is not recognized as a hybrid with any other species.
Below is a link to the UARK description of the species and cultivars.
When purchasing Dwarf Yaupon's, be sure the native species name is listed on the tags/markers and that it came from a reputable grower. A nearly identical dwarf Japanese Holly, Ilex crenata cultivar is sometimes labeled and sold as Yaupon Holly. The native Yaupon is more heat tolerant, but less cold tolerant, than the Japanese species
Here is a link that might be useful: Yaupon Holly, Ilex vomitoria
I think the answer depends on your point of view. Some native plant gardeners might think that only wild strains of native plants are suitable, others might use cultivars or hybrids of native plants and consider then native. Some people would consider only plants native to the ecosystem, plant community, county, or state in which the garden is located, other gardeners would use any plant native to the US, or to North America. I think you should know where the plants come from, and know what the parents of a hybrid are, then make your own decision whether it is native or not.