In direct answer to your question, no it would not make sense to have one disk at a different size or speed to others.
RAID will mirror/spread your data across multiple disks to create one logical volume. The upside is that your volume can sustain one or more disk failures (except RAID 0). When the array is created, if you have disks of differing sizes it will only create a volume based on the size of the smallest disk. For example if you create a RAID1 array using a 120GB disk and an 80GB disk, the resulting logical volume will only be 80GB.
If you're looking at RAID10, then your useable capacity will be (n/2)*Smin where n is the total number of disks you are using and Smin is the smallest capacity drive.
Although the principle isn't the same for drive speed, you will notice an impact if some of your disks are slower than others as the data is mirrored at write time, depending on your controller.