A flame angel would be fine in there imo
. Others may say not. I have a coral beauty in my biocube and he is happy as can be. It is generally the larger angels who are prone to nipping at corals. My advice would to just keep an eye on him. I've never had a problem with my coral beauty. Every once in awhile I think he's nipping my corals but he is just pulling off algae. He is very peaceful. If you want a flame angel you will need more rock though. They need hiding and swimming spots and also a decent amount of rock to graze on.
With the new rock you would have to go about curing it before you add it to your dt or you will have die off, meaning a mini cycle or even a full cycle again. Let me find a link to a good guide that lays out how to cure live rock and I will post the link here for you