The link about auxiliary water sources is about cross connection contamination, not about that you cannot have the systems connected.
“Make sure that the public water system is adequately protected by approved and regularly-maintained backflow prevention assembly(ies) at water service meter(s) to any site using an auxiliary water system.”
http://www.austintexas.gov/edims/document.cfm?id=191199
Table 603.5 tells you what kind of backflow prevention you need for the various cross connects.
I met with Austin Water, and according to them you always need one RPZ directly behind the meter, regardless whether the well is connected or not, just in case somebody does connect it. An “air gap” is only permissible for a rain water collection system. The typical configuration would be: utility shutoff, meter, customer shutoff, PRV, RPZ. If the well is connected and feeds an irrigation system, Austin Water also requested a second RPZ between the irrigation and the potable water, behind the first RPZ, instead of the double check. The RPZ is mounted one foot above the highest sprinkler head. However, you see them typically mounted next to the meter. Not all well owners I know have the second RPZ, and passed inspection anyway. I asked about this, and they said that health hazards are more strictly enforced, and an existing double check may be seen as “prior existing”. Supply pressure in our neighborhood is 150 PSI, hence it would be very unlikely that a well pump could backflow. This is generally about minimizing risk and I think the large concern is bacterial contamination. RPZs and DCs are very similar devices, only that the RPZ has a “reduced pressure zone” between the two check valves. In case of a double failure of these valves, a third drains to ground. If RPZs fail, they typically fail very dramatically with full supply pressure. You do not want to mount one in a basement.
Austin Water did not ask for an RPZ on the well itself. The drawings I saw have typically a plain check valve close to the pitless, or house entrance. There are also a couple of check valves in the riser itself, every 200 feet or so.
You do not have to be a licensed plumber to do the job. The homeowner can also do it.