How to design partitions for SSHD?

In a hybrid SSHD all SSD access is integrated with the hard drive's firmware and should not be controlled by the OS. You will be safe to just use, partition and format your 500 GB hard drive and rely on the engineers of that drive to give you optimum performance from the integrated SSD cache.

Also see Is Seagate's new "FAST Factor Boot" technology compatible with Ubuntu? for a bit more background considerations.

If you're concerned about excessive read/write with the /tmp directory, you can make it so that /tmp is a filesystem stored exclusively in RAM as per this comment: How is the /tmp directory cleaned up?

I've never heard of permanent writing having adverse effects on an SSD - it's excessive writing that really has the effects.

In your case I think your best bet would be to put your main / onto the 8 GB SSD (as opposed to the 10GB you normally use) and put the rest of your partitions on your HDD, as you typically did.