This is my opinion only, and I read your identical thread as well.
For the spotted cardinal fish- some people think they're better off in 3s or more but others have kept them by themselves so it's up to you. May be hard to feed at first.
I would get
either the bicolor pseudochromis
or the royal gramma. They may fight since they're identical looking.
I like dwarf flames. The sixline would be cool too and they like lots of LR.
The dartfish tend to jump sometimes so be prepared for that.
I wouldn't recommend the yellow tang (from your other thead). I know some have tried it but I'm glad you decided against it.
The hawkfish may eat your inverts, especially shrimp. I'd be a little nervous with the butterfly and their long streamers since other inhabitants may think that's a nice moving long snack.
The eel I can't help you on- never had one, never will. My buddy had one and I would bury a PVC pipe under your sand for it. I've also seen sites with this configuration and the eel seems to utilize it.
The mandarin- not for at least a year down the road, or when you have countless pods for him/her to eat. How much LR do you have?
Sea horses aren't usually recommended for reefs but have been done (Germanman has done it). I'd highly recommend researching this beforehand to find as much information as possible. I've just been researching them lately as I want to create a tank just for them. Apparently they need to be fed a few times a day at first, and should not be in competition for food with aggressive tankmates.
I'm sure the others will be along with their recommendations shortly. To me, picking the livestock out is half the fun.
