He still appears to be Mithrax sp.

Cool thanks!
The claws don't look super threatening so I think they should be friendly.
The tank has been up for almost three weeks now and I haven't seen ammonia or nitrites but the nitrates appear to be rising slowly. The AC got turned off without me knowing and the temp jumped to 81.5 which seemed to start the red algae spreading to the formerly dry rocks and I haven't seen any other colours yet. The crabs are constantly picking at the rocks so they seem to be keeping that under control for now. Thinking of adding snails soon...

Added 5 snails and they quickly got to work... Some green algae started sprouting from the sand in the lower flow areas... Looks kinda like grass. I rather like it. Did a bit of reading and I think I've decided on a neon goby, a clown, a cardinal and a royal gramma. Not sure what the ideal order to add them would be... Probably the clown or gramma first. Likely this weekend after a good water change if the ammonia stays at zero and the nitrates keep slowly rising.

Added two cardinals and a bicolour angel. I think the angel has ick. He's breathing really heavy and has orange spots on his head and along his gills. He just started rubbing against the rocks and he will swim up to the crabs to try and get them to pick at him. I don't have a hospital tank... Would neon gobies help at all or do I just need to medicate the poor guy? I've had him for three days and he just started showing symptoms last night. I feel so bad for him now. He was super happy when I first put him in but I just wish there was something I could do, I'm just worried about making things worse.

There is nothing you can do in-tank to treat ich. Try feeding garlic soaked nori, that often works to increase resistance, but once it's in there, it's there for good unless you remove all the fish for a few months.

Yeah.. sounds like I'll be setting up a hospital tank

I'm starting to think maybe my test kit is broken. Rather embarrassing considering I work for Mars Petcare... I've had three fish in there for a week now and still not even a hint of nitrates. The angel seems rather sick and I'm quite worried about him,but the cardinals are doing fine so that makes me think it's stress related. When he isn't hiding he's flashing and he keeps getting the green emerald to pick at him. I'm starting to think my only option will be to set up a hospital tank and give him some meds once I get a proper diagnosis.

Are you sure the Angel doesn't have Flukes? They are very common in Dwarf angelfish IME. That red algae, can you blow it off the rocks easily with a turkey baster or is it stuck on the rocks real well?