Dacotua

FYI: Most plants, including mosses are grown not submerged. When you put them in your tank they will ALL MELT, but they will come back with submerged leaves and such. Even Java Moss that is submerged for the first time will melt until it rebounds and grows in its submerged state

You may be over thinking the whole situation. In nature, many of the lakes, ponds, etc... all have tannins. Usually only alpine lakes are as clear as people think aquariums should be. I have several aquariums and my "Dark Water Tank", has Java Moss, Java Fern, Anubias and some Crypts in it, and they are all living just fine.

I would not do CO2 on any tank smaller than 20 gallons. If you don't monitor it correctly, you'll CO2 poison all your fish.

I would not put CO2 in a 5 gallon tank. Your sponge filter would be fine.

How big is your tank

Depends on how long you have had your tank up and running. I have a very heavy planted aquarium that has been set up for over 4 years (I have a sponge filter and a Fluval 307 Canister). I wouldn't be afraid of adding 30-40 Cardinals to my tank in one shot. (The amount of beneficial bacteria and the amount of plants, it really would take a lot for me to get an ammonia spike. The floating plants along with all the other plants just remove the ammonia, nitrite and nitrate out of my tank. I have been testing my water and I barely ever have any nitrate in my water. Plants are your friend.) I believe I could get away with doing water changes once a year if I felt like it.

You should never have a canister filter not prime itself. The motor is in the water. You can buy HOB filters with motors in the tank also, never have to worry about it not starting.

Go buy some cuttlebone (Sold in the bird section of your pet store, or amazon has them listed.) Just put a piece in your aquarium and you'll be fine (You can even put it in your filter). It will slowly dissolve in your tank and the snail will get its calcium. I do this in my tanks for my shrimp but the snails also seem to be healthier for it too.

HONESTLY: Do not clean your HOB filter until you see the flow start to degrade. Only then, its time for you to change the filter or clean it. I'm not a big fan of "cartridge" style filters. I prefer filters that you can just add sponges and biomedia. But since you just cycled your aquarium, I would not change the filter for at least 2 months. Let the inside of your aquarium get a nice colony of bacteria before you consider change the filter media. If you are considering a Sponge Filter, Put that in your tank now and leave your old filter hooked up for at least two months. Adding a new filter while leaving your old one will not overload your system. FYI: Beneficial Bacteria grows on anything that's in your aquarium. The glass, the Substrate, The rocks, etc.. It doesn't only grow in your filter.

You can always get a tube the same diameter as your updraft tube. Cut the 2nd tube to be 1" long and attach it below the top plate (inside the filter). This will force all the air into your updraft tube. Just make the tube just long enough to make this happen.

I used to use undergravel filters years ago. Do they work well? Yes they do. Do they have a downside? YES. If you ever have a problem with your undergravel filter, you have to rip the entire tank apart to get to it. Cleaning the undergravel filter is not a option unless you, again, rip the tank apart. You will still see the tubes going into the filter, you can get fish underneath the filter somehow, and you can even crush the plastic trays by the pure weight of the substrate and any decorations you put on top of it (Like large rocks). Also some fish / snails, love to burrow into the substrate and that could cause you problems. Also live plants will also be an issue if you choose to put them in your tank. If you ever add root tabs, they can be more "exposed" and release into the water column a lot easier than in tanks without the undergravel filter. I found I would rather just use a canister filter and be done with it. You'll still see the Tubes going into the tank, however you can clean this filter a lot easier. If I don't like the canister filter and I want to remove it, its easy. I would rather have two sponge filters in my tanks these days than having a undergravel filter. Again, I can easily remove these if I want to switch to something else. The undergravel filter, to remove it, did I meantion you have to tear the tank apart? JUST A FYI: Adding a coarse sponge to your outlet tubes will not stop fry from getting into those tubes. I have a coarse sponge on my intake tubes of my canister filters and everytime I clean my canister filters, I find fry & shrimp inside the canister filter. They can and will find a way in if they want.

In my personal experience, whenever I added leaf litter to my tank (almond leaves), the shrimp population decreased. If I removed all the leaves, the populations not only rebounded but exploded. I currently have two tanks, one tank has visible tannin's (Lots of driftwood, low light plants and almond leaves). In that tank, the shrimp seem to just die off quickly. In another tank, I have a lots of driftwood, but no almond leaves, lots plants and the shrimp population has absolutely exploded and thriving. Both tanks use the same water source, both tanks have live plants (Planted and floating). Just saying in my experience, If I add enough almond leaves to tint the water, the shrimp die off. If I don't add any leaves at all, my shrimp population has a massive explosive growth.

Every Nerite snail I've owned would not touch any wafers. They just ate the biofilm/algae off everything in the tank. (Want to feed them a little more, leave the lights on a little longer).

Your cycle still has not crashed. You are "Over Stocking" your tank in relationship to the biological filters ability to process waste. A crash is when all your bacteria dies and you have to start over

Your 407 definitely will keep up with your 55 gallon tank. Some people just run a sponge filter with a 55 gallon tank. Truth is, most people really over filter their tanks. IF your tank is algae infested its because your not balancing your lighting time (Having lights on like 12 hours a day or more) on the tank or you have it next to a window or something like that.