All great questions! Let's take them one by one.
1. Yes, you are correct in that both good and bad bacteria are affected by UV Sterilization and that the microorganisms must pass by the UV Bulb in order to be killed off. Any bacteria that are attached to the surfaces within the aquarium or filter will remain unharmed. The second part of your question is also a yes. The primary function of our UV products is to act as an aid to inhibit algae growth. If you are establishing the bacteria in a new tank set-up, then UV Sterilization is not necessary since there should not be any algae present in your tank.
2. The Aquatop PSE-1
is primarily a Protein Skimmer with the added benefits of UV and a filter. If you are only looking for a UV Sterilizer, Aquatop recommends the UVE-7
or the SP
Internal Filters with UV.
3. Yes, Protein Skimmers are primarily used in Saltwater setups due to the excess organics that accumulate on the surface on the water. They are very rarely used for Freshwater setups and would only be used under special conditions.
Hope this helps!