Are you sure you want to enforce it like that?
Because all other crypto-currencies don't have a hard limit. Bitcoin blocks could be found 1-2 minutes away from each other and that's allowed. You should control stake blocks off of the difficulty it is too mine them.
It's based off of hashing right? It's luck and the difficulty factor determines when the blocks are found. Having Pow, PoS, one after another would be very unlikely.
Well, it's the easiest method when blocks are all the same difficulty and people may all have them at the same time (very unlikely, but possible). Bitcoin blocks are fine being found close to one another because they actually represent some large amount of work -- all stake coins have is stake to back them. Regardless of the speed PoS blocks function at, PoW at 15 blocks per hour would be steady.
If you had (12/129600) of the current total number of coins in the network and a merchant who accepted funds after 6 confirmations, you could spam 6-12 blocks quickly after waiting three months and try to attempt to
double spend like this. The probability of such an attack succeeding is extremely high, and the cost to the attacker is virtually nothing.
I'm not sure 100% how PPC deals with this.
So, you need some means of mitigating this. In the Bitcoin network, the likelihood of getting enough hash power to perform such an attack (one hour of forked blocks) was extremely difficult, but it's very easy with stake blocks.
You also can not protect the network the same way you would with BTC/LTC for stake coins (by difficulty) because someone could simply hoard many coins, wait for the right moment, and engage in the attack. There's no guarantee stake blocks will be claimed at any particular time under the current system, and I'm not sure how you'd implement them to do so other than restrict the rate at which the network allows them to be generated.
Edit: Another option would be to force stake miners to wait until after a PoW block is mined before sending out a PoS block, so that PoW and PoS blocks are all mined back to back.
2nd edit: If you put PoS blocks back to back with PoW blocks, you can actually more easily attack the network because you now only need half the magnitude of your PoW attack vector. So, PoS in one sense protects the network, but in another sense destabilizes it with a small percentage PoS, small percentage PoW attack vulnerability. I haven't really seen this addressed anywhere by PPC, but thinking about it now makes perfect sense.