Shallow well pump systems

Can someone explain the different pumping systems, tanks tankless etc... I have a cottage with a shallow well probably 30 feet. It currently has a 1/2hp jet pump and ~25 gallon tank. The tank is full of rust which in turn is coating the UV sterilizer bulb. I am going to replace everything. I noticed That jet pumps are sometimes call convertible, what does that mean? Some wells I have seen have 2 pipes in them, what system is this? I would like to go with a fibreglass tank and my local well driller says a 6 gallon size is plenty, why was there a ~25 gallon one put there originally? What about bladders, do all tanks have them? Some jet pumps are stainless steel, is that the entire pump housing or just the shaft. I have some apartment buildings and the domestic hot water system has to have a bronze circulating pump so why is drinking water from a well allowed to be pumped through a rusted cast iron jet pump? Also I see some pumps listed as not needing a tank, why is this. When are submersible pumps needed? Thanks for clearing up some of this for me.
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I imagine that means that if you have a very shallow well, you can use a single pipe, and if the well is deeper it can use 2 pipes. For example my parent's cottage had an artisian well (flowing like a spring), so all it needed was a single pipe jet pump to build pressure. Since it was always flowing we did not have to worry about purifying it, although, it was very hard (limestone).
You can only lift water about 30' with a total vacuum, so if the water level is that far down, you cannot simply suck it out of the ground (you would have to push it out with submersible or 2 pipe jet).

Some iron is good for you (needed for red blood). If a pump is stainless steel, I would think that includes all the parts exposed to the water (not necessarily external parts). A jet pump needs to be primed if totally out of water (or if drained for winter). A submersible pump does not need to be primed, since it is underwater.
I would be skeptical of a tankless pump (where would thermal expansion from the hot water heater go), and even a 6 gal tank sounds rather small (pump would cycle on/off 4 times more often than 25 gal). A bladder makes sure that the tank does not lose its air charge even if shut down or system is drained (at least until the bladder wears out).
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