Fresh water your hard pressed to find a 'prettier' fish then the male beta (many varieties and colors) undemanding and only aggressive to other males of its own species give him a girl and there's a pair.
Thanks lauraleelbp! Too bad about the crayfish, and thanks for the pointer about the clams.Golden Wonder Killies or Hatchetfish would be good for the upper part of the tank as long as there are absolutely no gaps in the top for them to jump out of (otherwise, they will for sure).
Male guppies also could work.
Definitely 2 pairs (M/F) of Rams. And be sure that your tank is 'scaped in such a way that there is lots of hardscape and plants staggered around for them to mark off clearly defined and separated territories, and blocked sight lines. As long as they can remain out of sight of each other (especially when spawning) a 55 gal SHOULD be big enough for 2 pairs.
I wouldn't get 6x SAE for your tank. 2-3 would be fine. SAE sometimes get aggressive/territorial with each other as they get older.
The vast majority of crayfish species would eat your fish (they catch them at night while the fish are sleeping). If you want a cray, it really should be in a species-only tank.
Freshwater clams also would be a bad idea. It's extremely hard to keep them properly fed in an aquarium (unless you want to culture green water in the tank so thick you couldn't see the fish), and you'll never see them anyways as they'll burrow down into the substrate. Which also means you won't notice it when they die down there of starvation, and pollute your tank.
Most of the time when fish spawn in a community tank, few if any fry survive. My rams spawned every 2 weeks for a few years in my 90gal but I never had a single fry survive- they all got eaten! So I really wouldn't worry about that, if spawning is what would keep you from trying Rams.Are there any "centerpiece" fish that you can recommend that don't need caves, territory, or special needs? I'm not sure if I'd like to foster spawning either. Sounds like a reason to spend a lot more on refugiums, food, etc.