For RAID to work effectively, all drives in the array should be identical (manufacturer, size, firmware revision, controller). In your case, only the size is a common factor. That is not to say that mismatched drives won't work, quite often they will but at the cost of actual vs theoretical performance. In other words, if one drive is slower at processing information than other(s) on the array (perhaps due to different firmware or cache size), then the rest of the devices on the array will have to slow down accordingly. Sometimes, this can be significant.