So i am converting my 150 freshwater over to saltwater!!! I am super excited, I'm amped to get a lionfish! (fuzzy dwarf to be exact). I am also very interested in getting a longhorn cowfish, and my girlfriend really wants one of those stripe-eyed soldierfish (big eyed squirrelfish?)
I'm not so sure about the compatability of all of these critters, I am going to get the smallest lion I can so that I can even consider it having tankmates. Any tips or suggestions as to combinations of cool fish? other fish I am very interested in include: porcupine puffer, mandarin dragonets, frogfish, princess parrotfish

Might want to skip the mandarin goby and the princess parrot.
They are beautiful but not easy.

yea if i get a mandarin it would have to be many months later, I'm not sure exactly how long it takes to let copepods become prevalent enough to sustain one.

The Lion fish is awesome for a display fish. Just understand that anything thats is the same size as your Lion or smaller is "on the menu". The Mandarin Goby would do nice in a non aggresive tank that size after the tank is well established which would be 9-12 months at best. Getting these beautiful fish to eat frozen preps is a real challenge under the best of circumstances. The Cow fish is very interesting indeed but its really not a good fish to have except for the experienced and prepared hobbist. They can also cause a tank crash due to toxic secreations when harrassed, injured or they die. Not a fun situation. The Parrot fish is a no go in the home tank. Thier eating requirements arent easily met in the home aquarium. Frog Fish are sweet fish unfortunately their appitite is unmatched. If it is the same size or smaller its a done deal as it will disappear for sure. These fish are eating machines and will eat as long as food is available, so thier food has to be rationed to them. They dont do well in tanks with aggresive fish that will nip at them, tanks with halides lights or tanks with brisk water flow. I have found that these fish do very much better is a smaller single species only tank. But without a doubt they are one of my most favorite fish bar none! Porcipine Puffers or Puffers in general are pretty decent community fish in an aggressive tank with fish that dont have flowing finnage. They also have a tendency to develope interesting personalities and interact with the aquarist often time well. Not a good addition to a reef tank though as they can do some damage to corals. They will also polish off your clean up crew in no uncertian terms, eating your decrotive shrimp and snails. The Squirrel fish are great for a tank this size and in a dimely lite tank with lots of caves a shoal of 3-7 will provide hours of veiwing pleasure. They are nocternal for the most part but in a dim lit tank will often "come out and play".

For a really nice display of schooling fish you might want to consider Anthias. Just try to get one male and several females. These are very active schooling fish, that are easy to care for and very colorful. They will also do well with corals too. Just a thought...