Personally I would hold back on adding to many cichlid species in a 55.
I think the angels and Bolivian rams is enough. I also think you will get more out of your Bolivians and angels without adding more cichlids to interfere with them.
I don't think the tank will be lacking either,Bolivians and angels are both great interesting and entertaining species.
I would add more fish though. Black widow,black phantom, emperor and bentosi tetras can all be found together in the wild with Bolivian rams.
This is the direction I would take the stock,could be a nice little biotope.
I am glad you are considering this advice, OP. I love my (75 gal.) Discus/rams community, I have Corys, a bristlenose and a pair of Apistos in there too. I wanted some Rummynose tetras but it seems like a good mix, always fun to watch and everyone is thriving, so I don’t want to upset the balance.
Hatchetfish are a great choice, but I am personally partial to the idea of a big school of Rummynose, like 12-15. Unless you want to end up with fry, anyway, in which case I am a believer in dedicated species breeder tanks.
By the way, that’s my first aquarium, ever... I am nothing if not intrepid.