Is it safe to dispense chemicals in the skimmer?
Some chemicals cannot be manually added safely to the skimmer because they would produce conditions too harsh for equipment downstream. This is primarily true with acid since once it exhausts the pH buffering of the pool water (such buffering closely related to the Total Alkalinity level), the pH will crash making the pH too low. Trichlor pucks without water flow can similarly crash the pH.
The WaterGuru TREAT system adds its chemicals very slowly in a manner that is safe to downstream equipment. The acid is dripped in a few drops per second, far far slower than could be added manually – WaterGuru dosing can take an hour or longer, if necessary. The pH of the water at the pump during dosing has been measured to be safe to equipment and both copper and magnesium plates, weighed with analytical balances and their surface examined under magnification, have been used in pump baskets/strainers to show that there was no loss of metal beyond that in normally chlorinated water (I did those experiments myself).
As for Cal-Hypo, like all hypochlorite sources of chlorine it is higher in pH and the dosing is very slow at roughly 1 teaspoon per one to one-and-a-half minutes, again taking an hour or longer as needed for dosing.
The dosing from WaterGuru TREAT is analogous to what is done with peristaltic pumps in automated dosing systems in the pump room in commercial/public pools and some residential pools. Those systems are plumbed after the heater because they do not have a long enough pipe run to thoroughly mix and dilute their chemical injections. WaterGuru at the skimmer has the skimmer turbulence and a fairly long pipe run to the equipment and additional turbulence from the pump basket/strainer. The chemicals are diluted and mixed and the Cal-Hypo mostly dissolved. Whatever Cal-Hypo is not fully dissolved by the time it reaches the pump is caught in the filter where it continues to dissolve. Filter media, including cartridge filters which use polyester, have been tested with solid Cal-Hypo granules and found to have no negative effects. PVC pipe is very resistant to both chlorine and acid. The most sensitive parts in the pump room are pump seals in pumps and copper piping in heat exchangers in gas heaters.
Finally, the WaterGuru TREAT system has a water flow sensor so does not dose unless there is sufficient water flow.