Moss is present in their natural habitat so definitely doable. Not sure about plants as I've never tried it. The question might actually be, can plants handle the same conditions as the zebras require? I would guess that not many plants could live with next to no nutrients in the water column unless they had access to plenty of nutrients in the substrate. I don't know if it is possible to have lots of nutrient in the substrate without it leaching into the water column. Given the susceptibility of zebras to high TDS, I wouldn't risk it.
With regards to crossing with crystals - to my knowledge that hasn't been tried and I would suggest it would be very problematic (but not impossible) for three reasons:
1. I don't know of any crystals being kept at the same conditions as the zebs. My understanding is that crystals have trouble moulting if KH/GH are too low and zebras require super low TDS, GH and KH. This may change in the future as we figure out their requirements for successful keeping in aquariums and longer term breeding, but for the moment the requirements for both shrimp are miles apart.
2. Diseases carried by crystals are likely to adversely affect the zebras because zebras live in low TDS which has a low bacterial count and appear to be susceptible to bacterial infections when kept at TDS above their natural habitat (from my own experience), which you might have to do to keep both the crystals and zebras together. Once again, this might be overcome with time as we breed zebra strains that are better suited to life in aquaria.
3. This is an educated guess - it might be difficult to get zebras and exotics to mate/cross because, given the geographical distances between them, its likely that they separated into separate species a long time ago. In general, the more time that species have to evolve into separate species, the harder it is for them to cross. For example, its virtually impossible for a dog to produce offspring with a cat, despite the fact they are both mammals and evolved from a common ancestor some hundreds of thousands, if not millions of years ago, whereas dogs can mate and produce offspring with wolves. Dogs became a separate species from wolves when they were domesticated some 10s of thousands of years ago. Supporting my theory is the fact that crosses between Caridina shrimp have (so far) only occurred between species that have nearby geographical locations and therefore should be most closely related in the evolutionary sense. Prime example is the tiger and crystal crosses. these two species have been found in the same river systems. On the flip side, the genus Caridina might have only evolved fairly recently, meaning that pretty much any Caridina shrimp might be able to cross with any other Caridina shrimp and the only reason that we haven't seen these is because nobody has tried it yet. Given the amount of successful crosses that have been done (I believe there are at least 5 successful combinations of crosses between different Caridina shrimp species, plus 1 or 2 successful crosses with Neocaridina species), this may be true, but I don't think so because once again successful crosses have only occurred with shrimp from similar geographical locations. The other thing to consider about zebras specifically is that they can occur in the same river system as a very similar looking shrimp called Caridina confusa. If these species do not crossbreed (they might, but to my knowledge they don't), then there might be a mechanism that has evolved to stop them crossbreeding. One possible mechanism is the shape and appendages (hooks and spines ect.) on the males 'penis' which physically prevents the male from mating with a female of a different species. IF zebras have evolved to avoid mating with C. confusa then they are possibly less likely to be able to physically mate with crystals too.
We wont know with 100% certainty that crystals and zebras cannot mate successfully until someone keeps them together, but that wont happen until we overcome some of the issues presented by keeping them together - 2 of which I have outlined above.