So I think it is a matter of use case and personal taste. IMHO, just use one Pool. If you're going to replace the 5900rpm drives anyway over time anyway. I assume you run things over a network. As most are still running over 1Gbit networks (or slower), even the 5900rpm drives won't slow you down.
I've never used an SSD Optimizer plugin but yeah, it is helpful for writing, not reading (except for the off-chance that the file you read is still on the SSD). But even then it would need a datapath that is faster than 1Gbit all the way.
What you could do is test a little by writing to the disks directly outside the Pool in a scenario that resembles your usecase. If you experience no difference, just use one Pool, makes management a lot easier. If anything, I would more wonder about duplication (do you use that?) and real backups.