There's no reason whatsoever that the 7200RPM drives won't work, unless it's a firmware issue. Systems normally don't bias against drive speed, only drive firmware. And even then, rarely. (I wrote an article about it... http://lowendmac.com/ed/herlihy/08ph/upgrade-options.html)

Try what I suggested on one of the drives, and report back.

http://discussions.apple.com/thread.jspa?messageID=6237789

No mention of eMacs biasing against drive speed.

http://lowendmac.com/imacs/emac-2004-1.25-ghz.html

This also affirms that 7200 RPM drives are compatible.

I'll put money it's your drives formatting. Follow what I suggested.