Mixing drives

[Actionable Mango]

I am considering a DS411slim with four 1TB drives. The two recommended drives in this size are Samsung HM100UI (5400RPM) and Western Digital WD10TPVT (5200RPM).

I've read that mixing drives is good in a RAID because it greatly reduces the likelihood of two drives going bad (in case there is a bad run of drives). I have read about certain bad batches from both Seagate and WD in the past.

So I was going to mix these two recommended drives in the 411slim, but then I notice they are two different RPM drives. Is this a problem?

I suppose I could run all the same drive model/RPM, but try to get different batches by ordering one drive each from four different places, but having two different manufacturers seems safer to avoid a bad batch or even an entire model line that turns out to be bad.