I would guess that they can't. From my experience, the larger the difference between nymph forms, the less likely it is that the two species will be able to hybridize. The nymphs of E. davidi and E. javanica are quite similar so I would expect the two of them to be able to hybridize. However, these are all closely related species in the first place so I would err on the side of caution and not keep them together.