Yeah that sounds like it might work. Using sponge to block the intake holes should be enough. Whatever you use, make sure it is easily removed during tank maintenance days, so you can clean it. Pulling a sponge insert out and swishing it around in water to clean it is the easiest. Minimise your chores, so you have more time watching the shrimp.
That's fine. It's not your plants you need to worry about. Worry about the likes/needs of the shrimp first.
Shrimp prefer a slower flow.
Correct. The ferts themselves do not kill shrimp, but ferts change the water parameters. And the constant change can upset the shrimps health.
Stones can change water parameters - KH increases and thus pH also increases.
If you find your KH and pH rising and you need to battle it rising then look at removing the stones.
Flourite is an ok substrate for Neo Caridina. Hopefully Neos were on your plans for this tank.
That's normal for a cycling tank. As the bacteria convert Ammonia to Nitrites and then to Nitrates, they cause pH, KH and GH to drop.
This is fine while the tank is still cycling. Keep your pH above 7 using tap water (dechlorinated). You can reduce pH, GH and KH after the tank has cycled to match the shrimp's requirements just before adding them.
Fish are a different ball game. Be prepared to re-learn all new things when keeping shrimp.
I bet you never needed to worry about TDS with fish. So a tip from me is to buy a TDS meter. You can get a cheap one for now (Amazon or EBay). It won't be super accurate, but that doesn't matter. You just need it to be consistent, even if it is consistently reading 10ppm off. Knowing your TDS (Total Dissolved Solids) in the tank is very helpful to keeping healthy shrimp, and the TDS meter can be used to improve your water quality for the fish tank too.
RO is undoubtedly better because you are in control of what is in the water. But of course there is an initial cost of purchasing the unit. Check out the Water Parameter subforum and read topics on RO water. Weigh up the benefits vs the cost and then maybe you can make a decision. (RO water is much better than tap) Even Rain water is many times better than tap water.
Not the best idea when the tank is cycling. Heater is required at this stage of the tank's setup for bacteria growth. Think about it ... do bacteria grow faster in a warm wet environment or a cold environment? - Crank the heat up to 28degC. unless you have lots of time to waiting around for bacteria to grow in the cold.
The bacteria needs 4 things to grow...
Neutral to Alkaline water (ph 7.0 up),
food (ammonia),
oxygen (well aerated water), and
heat (heater).
Remove one of these and they don't grow or die completely.
Tap water can provide pH 7 & up water with some ammonia in it, so all you need is to provide water that is circulating and heat.
If you squeeze the gunk from your fish tank's filter into this new tank, you will seed it with millions of beneficial bacteria, thereby reducing the cycling time drastically.
That's how I cycle a new tank. I use tap water initially, squeeze in the gunk from another filter (from a known healthy tank), turn the heat up and wait.
After the tank is cycled, I empty 99% of the tanks water and add RO water adjusted to the parameters of whatever shrimp I am keeping.