Maybe we are not understanding each other.
If too small of an injector is used, the orifice will be smaller then the item that was intended to regulate the refill rate to the brine tank. This will have nothing to do with whether or not hard or soft water water is put into the brine tank. It can affect the systems useful capacity during service if the brine tank does not receive enough water.
Most online resellers do not actually sell stock, they use companies like mine to drop ship. We have rid ourselves of most of the online dealers since their only goal is to be the lowest price and quality has to suffer obviously. The use of the lowest cost mineral tank, medias, build components etc. Also many of the online companies run several companies from their moms basement so their knowledge is pathetic. If you read their websites, you will notice they all simply cut and paste each others information, technical data etc. Many of these companies are being denied the higher end USA made components since they keep pairing them to junk and deferring all technical calls back to the components manufacturers.
As to why the wrong injectors etc. are used... many of the OEM's don't care when they are being pounded for pennies in order to keep bad business. They simply turn and burn equipment at margins that are unable to accommodate taking the 10 minutes it takes to properly set up and program a system properly. They simply send them out "close enough".
As to the use of 00 injector, be sure to either time it yourself or use the supplied draw charts that came with the system to determine the proper brine/slow rinse cycle. Typically the slow rinse should be 3x the draw cycle. The fast rinse cycle should never be used to rinse the system, it serves a different purpose. If you look at the chart, the injector will flow over .25 gpm so it should be adequate, barely. The .125 would be a better choice but it is unlikely it would make a difference.