I'd probably go with the neons or rummies, which ever because both are great options. However, the rummies do get larger so to keep things looking larger than what they are in terms of scale, i'd probably go with the neons. Green neons are even smaller than the common neons so you can consider those too. I've always been a big fan of GBR's. So I'd say go with the first option.
Shrimp are cool, but CRS and other caridina shrimp for that matter (except the Amano) are more difficult to keep than Neos. Unless you really have RO water or a tap source that's really low in TDS, you'd probably find the shrimp difficult to keep and breed -- especially in a high tech tank with co2. I'm not sure if you're thinking of keeping a second tank (maybe down the road), but you can have the best of both worlds by doing a nano low tech shrimp tank. Those AquaTop high clarity cubes (from truaqua.com) are great for such a task. They're pretty inexpensive for an ADA-clone but quality tank IMO.