Hi,
commercially speaking, NO.
But if you have already bought them, I say that they are ok.
It is not completely true that these disks will go at 10K:
this is completely true regarding size: if you put in an array disks of different size, and the controller accepts it, the size of the single member is that of the minor.
Regard speed, the operation on these disks will be completed before the others, but becouse you are using
an array card, until all disk operations are not finished
the card will not send a I/O completed to the system:
numerically speaking this is the same what itsp1965 sayd.
An alternative you can do, but it depends by what you have already on the system and from the space you need, is to build a RAID1 (REAL 15K) with the new disk you bought, and install here operating system; then rebuild or leave yor RAID 5 on the old disk, for data and an half of the paging file.
With this 2+3 configuration, you loose 1 72Mb of size,
but you gain 15-10 rpm on the OS.
ciao
vittorio