Blennies: Parameter wise (salinity, etc.), yes. Requirements: Bicolors are herbivores, and need the appropriate foods. They'll relish all meaty foods you give, but veggies (seaweed, nori pellets, etc.) is required. I recommend them because they do keep part of the tank clean of algae, but they do still need food.
Shrimp: I'm not sure, but I'm pretty positive that the eggs hatch during night. This means no food for the fish unless you larval snag them or collect them out of the sump.
Snails: I'd start with 5 Asterinas, 10 Nassarius, 7 ceriths, and as many stomatellas as you can stomachella. They'll eat algae, and the Nassarius will scavenge detritus. Both Nassas and ceriths will stir the sandbed as well.
Pods: Gammarus-looking amphipods are good additions to the tank. I'm not too certain about other amphipods though. As for copepods, I like Tisbe and Euterpina, as well as those that come in on live rock with no coral attached. You're looking for harpacticoid copepods (benthic pods), not calanoid (pelagic). Some people like Tigger pods, I don't.
Firefish: I do only feel comfortable with 4 fish in a 29 gallon tank. However, unless you can absolutely, positively, ascertain that the 'pair" of firefish you're getting is established, I wouldn't chance it. Dartfish/Firefish are hard to come in male/female pairs due to the lack of physical distinction between sexes, and if you get two of the same sex (male-male or female-female), you'll probably end up with only one. Unless you want to go the ways of the cichlid breeders (buy 6 youngish ones, hope for the best in a larger tank).
