Your questions might get a better answer in the NAS / RAID forum:
http://www.tomshardware.com/forum/forum-32-324.html
AIUI, some important factors to consider when assessing a drive's suitability for RAID are whether it supports TLER / ERC (Time Limited Error Recovery / Error Recovery Control), whether it is affected by APM (Automatic Power Management), and whether it supports the HDD Activity signal on pin #11 of the SATA power connector.
TLER / ERC determines how a drive handles read/write errors. It does this by imposing a timeout limit on reads and writes so that a drive won't be dropped from the array if it gets stuck on a bad sector.
APM causes a drive to go to sleep after a certain period of inactivity. Waking up from standby can take several seconds, which may impose an unacceptable lag on data transfers.
The activity LED is not always implemented by the HDD manufacturer. In fact I have seen one Seagate model which didn't support this signal in one particular firmware version, but did support it after a firmware update.
In short, it may be that WD and Areca have tested different firmware versions and arrived at genuinely different conclusions.
As for mixing drives of different speeds, I think this is OK, but it stands to reason that your RAID's overall performance would be limited to that of the slowest drive. In fact some people suggest that mixing drives from different manufactures or different families potentially improves the reliability of the RAID.