feeding flake foods contributes to algae too because half of it gets dissolved uneaten. Plus rinsing your mysis shrimp under the tap with a brine shrimp net helps too compared to just thawing it and dumping it in.

As for testing phosphates it's a moot point unless you have an organic phosphate kit. Our hobby kits test inorganic phosphates. You could have zero inorganic phosphate on your hobby grade kit, but sky high organic and never know it. Organic kits usually run $250, such as the Hanna kit, which makes them out of reach of most hobbyists.

As for deciding on your clean up crew, snails are more likely to starve than crabs and shrimp. Go light on your snails (like 1 trochus or turbo per 10-15 gallons). Go for younger (smaller) snails too because most molluscs have a 2 year lifespan. Those giant trochus snails in the store might croak in a couple months. Crabs and shrimp you can just toss in food for them if there's not much else for them to eat.

Shimek's 500 aquarium invertebrates book has great info on snails and other CUC animals (including what might end up munching on coral heh). An addition to the library for only $20.

I had my water tested at my LFS store they have a Hanna Tester.
You can ask your LFS to run a test to see what they are.