On the squeezing the moss balls, I would recommend not squeezing but only gently swishing in a cup of water to id the of the particulates which might form. Also use a chop stick to rotate them a little every few days (or a week) to keep all side nice and green and good shape.
They require low light and are safe for all aquariums provided they aren't torn apart by rowdy fish.
They are indeed a plant just a moss.
They apparently were called Cladophora but it has a distinction in genus and called Aegagropila linnaei.
Wiki has a nice bit of information about it
If the moss balls have a decent light source like a full spectrum daylight lamp, they will produce some O2
with photosynthesis and provide oxygen for the 2 shrimp.
I have had a lost shrimp in a cup of moss in water, in really no light, survive for weeks because it got stuck in the moss that was removed from the tank and only later realized there was a shrimp in the cup!
Petco has Cherry Shrimp which sometimes are a nice dark red. You could get a male and female and see if they breed. You would need to do pwc
1-2x per week and feed a spec of food for each every 3 days.