For a new tank, that's pretty much the case, although its really just a guideline. What you really want is a well balanced crew of algae eaters, detritus eaters, glass cleaners, and sand sifters. Each cleanup crew member does one, some, or all of those to varying degrees. If I had a 90gallon here's what I'd reccomend:
20 Cerith Snails (Jack of all traits)
10 Astrea Snails (Glass and rock, diatoms, some algae, some cyano)
6 Turbo Snails (Love the glass, diatoms, some algae, some cyano)
10 Nassarius Snails (Sand Sifters, some glass, some algae)
10 Blue Leg Hermits (Rocks and sand, diatoms, algae, detritus, uneaten food)
6 Red Leg Hermits (same as bluelegs but some cyano)
3 Emerald Crabs (Rocks, big algae eaters, including hair)
1 Brittle Star (Sand sifter, detritus eater, uneaten food)
3 Skunk cleaners and/or blood shrimp (Rocks and sand, detritus and uneatenf food)
I go heavy on the cerith snails because they're small in size and eat the widest variety of nuisance items in the tank. Some people have different experiences with different CC members eating other things. In the end it'll come down to what works best for you, but that would be a good starter for your tank.