I think your luck with the rebels being aggressive and the Ptolemies and Parthians being passive really didn't have much to do with your starting. I've found that if you just move your troops out of Asaak and just let the Parthians march in without a struggle, it kind of slows down their momentum. I always give up the northern most towns above Baktria and the one directly west of them too. I split up all of the garrisons between the town southwest of Asaak and the one directly south of it. I think the extra garrisons in those two cities discourages the AI from attacking. I also build stone walls as soon as possible and get a bunch of archer spearmen when my funds permit.

I play on M/M though, so it might not work the same way on VH/VH. I think probably most of the AI behavior is just random.