Your tank aet up sounds fine. 55g is a nice size for seahorses.
I am not really sure about the lighting and the hard corals but I do not think it is enough light for them. I don’t keep them and do not advise keeping them with seahorses. I had a seahorse stung once, as a result her tail was paralyzed, she became difficult to feed and I eventually lost her. It’s just not worth it IMO.
Placing the corals high in the tank presents the problem associated with the horses hitching high in the tank. The risk of GBD is increased when horses hitch in the upper level of the tank.
Although experimenting out of the typical range can be fun and keeping unusual and colorful fish is very interesting IMO it is just not worth the risk. Fish that have been classified as difficult to keep or delicate are labeled as such for good reason and as I said before usually do not do well in captivity. Seahorses can be challenging enough without the addition of fish that are not considered ideal or are difficult to keep.
I hope you are planning on quarentining your wild caught tankmates. This is a must IMO. I always recommend adding the seahorses first and once they are eating well and established then adding your other fish.
I will warn you if you do have trouble with the Angel and Butterfly they will be difficult to remove and there is a good chance you will have to disturb your tank possible take it down to remove those fish should they present a problem. Something I have done on more than one occasion because early in my seahorse keeping days I got alot of poor advice and also felt the need to experiment and play on the edge. I no longer do it and do not recommend it as I said IMO just not worth it. This hobby is not nearly as enjoyable when things go wrong. It’s easier and more fun to get another tank. Also you should have a back up plan if the tankmates do not work out. Where will you put them? Can you return them to your LFS?
I prefer your plan for 3 pairs of seahorses with the Banggai Cardinalfish and Green Clown Goby. I find seahorses do best in groups as opposed to single pairs in terms of their feeding behaviors. I would go with the erectus. They are more hearty that reidi. As for tankmates you could actually get a pair of Gobies and Banggai Cardinals. http://www.inlanaquatics.com sells paired fish. I am sure these would be very interesting. They have many fish not listed on their stock list so an email inquiry may be prove beneficial. They may even be able to get you a Helfrichi’s firefish or one of your other choices. Scientific names would be helpful so they know exactly what you are looking for.