Ok, gonna break this up into sections. I'm assuming you mean Marimo moss balls (actually a type of algae).
Bought a 14 gallon tank kit yesterday. Until now, my marimos were just in a bowl on the kitchen table. I can already tell they're happier. We've gotten five Cherry Shrimp, two Fancy Guppies, two Mystery Snails, and one of those sucker fish things.
You got all of those the day after buying a tank? Unless you're seeding the filter with old media from an established tank, I doubt anything but the apple snails will survive, especially those guppies, they are not a hardy fish.
I have a total of 5 marimos. One about 4 inches wide, three about 2 inches wide and the last is about a half inch that came from a split. How many Cherry Shrimp should I have for them?
...you don't need shrimp for Marimo balls, nor do shrimp need them.
Is 75 degrees an okay temperature?
Yes, as long as this stays somewhat constant.
Can I safely put other aquarium plants in without slowing my marimos growth?
That's fine, Marimo balls need almost nothing to grow, however keep in mind that they grow very slowly.
What other fish would go well and get along with them?
In a 14gal, not much. You'll be kinda limited to smaller species, like the guppies you have. That plecostomus you bought (the "suckerfish thing") can grow to 3.5-4 feet
, you'll be better off taking him back to the store ASAP, as if you just got the tank yesterday, unless you give him food (zucchini, squash, etc), he will die very soon as there is no algae for him to eat. He will also outgrow your tank very quickly, and if you have plants, he will rasp on them and destroy them, also uprooting some with his powerful tail. Look instead for otocinclus catfish, they stay around 1.5". Keep them in a small school of four or so, and at least once a week put a slice of blanched zucchini/squash in for them to eat.
Best filter? Can we put two? Tips to keep it as clean as possible?
Do you not have a filter on it at all right now? If so, I'm surprised the fish lasted the night. Get an Aquaclear HOB (hang-on-back) filter, something that's rated for at least twice as much as your tank can hold (so 14x2 = 28, so look for something rated for at least a 30gal tank).
Without cycling your tank before adding any fish, the bacteria that convert ammonia to nitrite to nitrate are not present, ammonia will build up to dangerous levels in the next day or so and kill your fish, starting with the guppies.
You're better off to take them back to the store, buy a quality filter, ask them if you can have a small sample of used gravel from one of their tanks or a bit of media from one of their filters, put this in your system, and wait for it to cycle. Also buy an API Master Test kit (liquid, not the strip testers), and check the water each week (or several times a week) to check for the ammonia to spike, nitrite to spike, and finally nitrates rise to ~10-40ppm. Once this happens, the tank is "cycled" and you can begin to slowly
add fish, as to not overload these bacteria and harm/kill the fish.
Sorry to be the bearer of bad news, but if you just started yesterday, there's some stuff you need to know. This is something that most people think you can just do in one day, but it is a lot more involved than that.