I do believe that you won't get any uploading without being on WiFi. I'm not entirely sure if it requires being plugged in, but I would do that anyway.
ALSO ... be careful about referring to that as "synching" because that could give sone people the wrong idea about what is happening here with iCloud Photo Library. For one thing, it's not really "synching" the photos on your iPad with the Photo folder in the cloud. If it were actually "syncing", then what was on your iPad would be matched up with what is in the "Photos" cloud folder (seen in icloud.com). That would mean if you delete (let's say) ten photos on your iPad, those same ten photos would be deleted in your "Photos" folder in the cloud. And also, that would mean if you went to that folder in the cloud and deleted a different 10 pictures, those same ten pictures would be deleted from your iPad, as a result. "That" would be synching.
What's really going on though, is that it's just uploading the photos on your iPad to the cloud folder of "Photos" and doing nothing more with them, except to keep them there permanently ... no matter what you do on the iPad. You could delete half your photos on your iPad and it would NOT synch with your iPad and delete half of them "in the cloud" Nope! You can delete them ALL on the iPad and everyone of those photos that were "uploaded" into the cloud, will REMAIN.
Of course you "can" delete photos from the cloud, if you want. You'll just manage that from within the cloud, itself, if you want to do that.
Just ust think of the "iCloud Photo Library" as your permanent repository for all your photos, no matterwhatdevicethey came from ... and a repository in which any of your devices can "draw from" ... namely download from the cloud and put on your device.
AND THEN ... a real big WARNING for right now ... this is only a BETA SERVICE at the moment and if you use it now, you are a BETA TESTER, which means you should have NO EXPECTATIONS that any of your photos are safe during this beta testing period. You are merely "playing" with your photos right now, with absolutely no expectation that they will be preserved or safe or saved! You will only have that expectation when it is released to its FINAL form, to the public.