Welcome to the forum.
Unfortunately, PrE does not usually do well, with mixed formats. PrPro CS 5.5 does a better job at that.
I would do a quick test, with small Clips from the two cameras, and see how it goes. With the AVCHD material, I would first set up a New Project to match that, then add the HDV material to that Timeline, and test. If that does not yield good results, I would reverse the test, with an HDV Project, and Import the AVCHD into that.
I think that you will find it better to create two Projects, one for each format, and then edit them separately. You can then Export/Share one, say the AVCHD, to match the HDV footage. Then, you will likely have an easier time editing that Project.
For intermediate files, especially with a compressed format, such as AVCHD, I like using Largarith Lossless, or the UT Lossless CODEC's, but do not know if they work on a Mac. This ARTICLE goes into more detail.
Thanks Bill, that is very helpful.