I have always personally been against questions solely about Greek. I understand that there is a definitive link between Greek and Latin, but it just doesn't make sense to me that Latin Language Stack Exchange includes questions about a language that isn't Latin, and that the substance of said questions does not pertain to Latin in some way except for the fact that the language in question is etymologically related. I full-heartedly support a separate site for Greek, and if this site is created, all purely Greek related questions (i.e. those that don't include some relevance to the Latin language itself) should be considered off-topic. For now, we can retain our current policy, but if the site appears to be gaining momentum in this commitment phase, I believe we can begin our shift away from Greek. And, if the site reaches the beta stage, this is a clear indicator that we should begin to distance ourselves from Greek, as there is obviously no need for two sites that will accept Greek questions from the same time period and about the same substance. To be clear, Greek questions that directly relate to Latin will remain on-topic, but those that have a weak or non-existent link should not be allowed.
In summary, wait, but once the Greek site gains momentum, make purely Greek questions off-topic.
To further clarify, Greek should be declared off-topic once the Greek site reaches beta. For now, we should encourage Greek questions to be posted on the Greek site to support their efforts!