I have a 120 L cold saltwater tank with about 52 shrimp. You will find that bigger fish will harass the shrimp unless they have a very good place to hide. They will also eat anything that lives at the bottom of the tank, e.g. bash yabbie and small crabs.
Aiptasia is a hitchhiker...it doesn't bloom like normal algae does. The only problem I've had with my peppermint shrimp is they keep on taking the food before the coral or fish can eat it. Very selfish little shrimp LOL. But they are a great addition to any fish tank, whether they be reef aquariums or fish only aquariums.
I have 2 peppers in a very sparsely populated 225 L tank. They generally appear to be shy, hiding in the rocks. Bizzarily, curiosity regularly overcomes this apparent shyness. They will quickly visit my hand in the tank and start 'nipping' for loose pieces of skin! Entertaining for those visitors who think of a fish tank as only something to look at. Also they like cooked turkey around Thanksgiving - don't ask!
I bought three peppermint shrimp when setting up my reef tank. The live rock had lots of aiptasia anemones growing over it, the largest of which was stinging a small colony of button polyps. The peppermints ate the aiptasia in about 7 days because I didn't put any food in whatsoever, thereby eliminating the easy option, forcing the shrimp to pick out the pests.
These shrimp do a good job with Aiptasia, but only if the Aiptasia are small. You can get rid of the big growths by taking a squirt syringe filled with calcium, releasing it into the middle of the stem and that should kill them.
I had my first aquarium in 1967. I spent a whole bunch of years raising angel fish, and made more than enough to pay for my hobby. Switched to saltwater in 1986, have two reef tanks (280 and 510 L) that have basically been set up since then. I could talk about a lot of things, but since the topic is Pep shrimp; I've never had a problem with them bothering (within reason) anything in the tank except aptasia, occasionally they will nibble on soft corals, but they haven't ever really hurt anything in my tanks. Also they reproduce quite regularily if conditions are right. But mostly all the offspring do is feed other things in the tank (which is OK). Any creature that provides food in the plankton column is fine with me. In my experience, once you introduce them into your tank, the aptasias slowly go away. Direct relationship, I don't know, but it sure works. My final comment is: anybody in these pages that states an absolute is not worth listening to...my approach has always been: in my opinion, or in my experience.