1. There are no advantages to a 25 or 50 GPD system. The main disadvantage is it takes 2 to 4 times longer to make the same amount of treated water as a 75 or 90 GPD RO membrane so don't buy anything but a Dow Filmtec 75 GPD, GE Water 100 GPD (basically the same as the Dow 75 but rated differently) or the Spectrapure 90 GPD which starts out life as a Dow 75 but is specially treated to improve its performance.
2. The source really does not matter to the RO system as long as you have 40 psi minimum and softened water is better for the membrane than hard water but either treats fine. The difference is soft water will make the membrane last longer for a lower cost of ownership.
2.1 Chlorine or chloramines, it makes no difference as long as you use a single high quality low micron extruded carbon block. There are no advantages to so called chloramine carbons and some of the tests lately have been bogus and skewed. The EPA MRDL or maximum residual disinfectant level for chloramines or chlorine either one is 4 ppm or mg/L and a good carbon removes the chlorine portion and breaks down the bond with the ammonia which is removed by the DI, not the carbon as some try to make you believe. In truth the DI is the most important part not the carbon.
3. As mentioned in 2 above, 40 psi is the bare minimum to operate a RO membrane but 60-80 or higher is even better as the higher the pressure the more efficient the membrane becomes. I run 95-100 psi at 99.43% rejection rate myself.
Spectrapure has more information here than you may want to read but everything is there:
http://spectrapure.com/frequently-asked-questions
"Things you need to know before purchasing your system" covers most of it:
http://spectrapure.com/Reverse-Osmosis-General-Info
Besides that, they have every possible hook up configuration you could imagine:
http://spectrapure.com/hookup-diagrams
and manuals for all their products:
http://spectrapure.com/Product-Manuals
and a good educational section on how RO systems work and more:
http://spectrapure.com/technical-info
and even videos is you want those:
http://spectrapure.com/Videos
and here is an old thread on chloramines that still applies today:
http://www.reefcentral.com/forums/showthread.php?t=1735402
and another:
http://reefkeeping.com/issues/2003-11/rhf/feature/index.htm
Chloramines are well understood and not the problem many try to make you believe. A good carbon and better DI with good contact time is all it takes, not catalytic carbons designed for industrial applications with high levels of chloramine way above the EPA allowable level
They have not been the leader in the industry since 1985 for no reason at all, that 30 years of satisfied customers. Their basic $125 refurbished RO/DI blows most other systems away but you can always go more expensive if you are looking to extend filter and resin changes. If you are confused call them, they would be glad to explain the different systems.