I have a 120 and for the film and slight algaeal growth on the glass I have 6 Turbos and 6 Astrea's and they do an excellent job keeping up with the algae situation. ( Turbos have a rep for bulldozing tho so be careful your corals and such are planted or epoxied well as they can bowl over stuff)
Tho my situation may be better than yours as I am skimming hard and have excellent water parameters. So this amount of crew might not get you done. The rule of thumb is never written in stone, find what works in your system and stick with that.
Keep in mind that even tho the critters we call a cleaner crews are necassary and helpful they also produce waste also, which as you know = nutrients. What I am suggesting is only add what will get the job done and dont over do it.
But this should not be a problem once you get a skimmer up and running you will be ok.
My feelings on crabs is they are what are called oppurtunistic feeders. Ya one type may eat only XYZ for its main diet, but it is a fact that if you dont have enough XYZ for whatever is its primary diet is, it will feed on anything it can. Corals snails etc.
I dont remember seeing how you are addressing control of your phosphate levels but sooner than later this will become something you want to address. There are two fairly well known brands of phophate exporters for chemical means, RowAphos and Phosgard and a bevvy of others, After lots of reading and looking at reseach done on the two major brands noted I use only Rowa the other brand has been tested by some well known names in the hobby and the Phos has been shown to have aluminum in its make up that binds the PO4 the testing showed that corals, both SPS and LPS & inverts get very stressed with high levels of aluminum. Another way to control PO4 is using a fuge with Cheato or some other type of macroalgae. Then harvest on a regular basis to export the nutrients. My buddy uses Red Mangroves which kick butt but they need a lot of vertical space to grow. My partner has a 50 gall fuge in a fish room so no problem there.
What also helps is keep your feeding to a minimum, when you feed silvers, shrimp etc rinse them in distilled water first, this helps to wash away the PO4 found in these foods. Even the LFS brands of foods are known to add PO4 to your system. If you use mechanical filtration with activated carbon make sure it is a reputable brand. Some AC is known to leach PO4 back into your water. It is the way it is processed that causes this condition. What you can do is find a brand that is supposed to be PO4 free and then add some to a cup and add RO water to it and lets it sit a day. Then test for PO4. This will let you know if it is safe for your tank.
You know after all this typing I just thought that I dont even know what type of tank you are keeping
most of what I have shared is mainly reef tank husbandry but can be applied to FOWLR setups as well. Sorry for the long post. I just like to give back what was so freely given to me(knowledge) as it has made my journey in reefkeeping much more enjoyable knowing what works and what doesnt.