Firemouths are in general a pretty peaceful cichlid. though I have read stories of them being mean they are rated pretty low on the aggresion'O"meter.
I am not sure if they will nip fins of the angels but there is a good chance they will. The angelfish are usually held to be even less aggressive than the Firemouths and they physical makeup does not make them the most efficient combatants.
the firemouth is a member of the
Thorichthys Genus and has several relatives that are very similar if rarer.
I couldnt get a link to work so go to
www.fishbase.org and in the "Genus" text bar type
Thorichthys
then hit enter this should bring up a list of about 8-10 names all starting with Thorichthys.
if you click on the names it will give you info on each, there should be a picture in the top right. If there isnt in its place there will be a "try google images" button that will try finding pictures on google. (duuhh

)
I believe it would be worth trying a firemouth with your angels but keep a close eye on them. and make sure it is a young firemouth as an adult will likely be less forgiving.
my personal favorite from this Genus is the Aureus, it looks like a more colorful Firemouth (Meeki)
instead of your angels you could try to purchase a male and female firemouth and then have a school of tetras to add some motion higher in the tank and maybe some corydoras in the bottom tank.
btw your best bet at getting a male female pair is buying 6 juveniles and removing the extras once a pair becomes apparant.
I hope this helps, Please let me know where you go from here!