In Captain Planet did the kids have to be physically together to summon him, or could they do it from their own countries?
This depends on the range in which their ring powers (once released) can detect each other. If I understand the shown procedure correctly, they all shoot the power from their rings up in the air, where they find each other and combine. I'm not sure the furthest apart they've been in the series, but they don't have to be immediately adjacent. Because their power goes up in the air, I'd imagine they could be quite a ways away, as long as they coordinate the action (say, over 2-way radios or telephones). If I recall, they were all from different continents. Could they summon him if each of them were home? Probably, but it depends on how the power from their rings knows where the other powers are. Captain planet would show up at the junction of the ring powers.