They are easy to plumb in. (I think, but i build things)
75 GPD will do for your two tanks, remember when it's cold they slow considerably, your 50 GPD in winter may struggle to push out 25 GPD. Depends how cold it gets, ground water below 10'C really slows these things down.
All you need to do is drill two holes about 1/4" one on the feed one on the waste.
Plumb accordingly. The self cutter is useless, that's why I pre drill, care must be taken not to squash or dent the feed pipe and to ensure the grommets line up properly over the holes.
TURN WATER OFF FIRST! Use main feed not tank supply or you will have no pressure. (Normally kitchen and bathroom cold are direct from main feed)
I use a 75, it's good for 4 tanks but only just copes with the winter.
(Using normal containers only) 25 litres 1.5 hours in deepest darkest winter.
With a reservoir it's a bit different, you can just leave it on.
25 GPD will produce 25 GPD over 24 hrs at 24'c and 60psi, lower temps slow production, higher temps increase production. Same with pressure, you need at least 40 psi to cross the membrane, I think 60-65 psi is optimum, at least for my membrane.
(I used treated water for years, it's ok for a lot of fish, it depends on your tap water, the fish you have and those you want)
Test your tap water, if it's soft and nitrate free, assuming there is no Chloramine, you don't even need a conditioner. (24 hrs will gas out chlorine) heavy metal binding could be useful though. Conditioners have them.
Soft water you just need GH
powder unless you are reefing it. Hard water is ok but will need RO
to soften, it's an equal cut by % pretty much.
alone will kill your fish, you need to re mineralise it, a tap water cut is fine, I did it that way for many years and kept many things alive with that water.
Some of my fish have seen the whole transition from tap only through cut RO
mix to pure powder/RO
. They seem fine.