Probably a platy, i would not recommend neons as they are usually small in the shops and unless you keep the angels with neons from a young age they will see them as food, i do believe neons are part of an angels natural diet so i don't fully understand why keeping them from a young age helps at all. There is also the fact neons (and other tetras) have the potential for nipping if insecure or in a badly kept shoal.
About that temperament changing as they age thing. My angel was a real pushover when i added him but now, a few months on, he is beginning to chase all of his tank mates if they step out of line; if they themselves were not pushovers it would be a bloodbath. My guess would be that being cichlids, there is a good chance neither of your angels or your bolivian ram will back down, seeing how it is a small territory to keep all three in there is also a good chance one will have nowhere to run.
The least risky addition at this minute would probably be a platy, try to get a fully grown one if you really want to go ahead with it.