You can add an ssd just fine,
There is no such thing to my knowledge as a 3.5" ssd - you will need an [http://www.newegg.com/Product/Product.aspx?Item=N82E16812200193&nm_mc=OTC-Froogle&cm_mmc=OTC-Froogle-_-Adapters+and+gender+changers-_-STARTECH-_-12200193|adapter bracket].
Jumping pins is for fooling the thermal sensor on the new 27" iMac. not relevant for your imac.
MCE TECh makes an optibay that will allow you to use a 2.5" ata drive harddrive in place of the ata optical drive. Powerbook G4 optical drive's are the same size and shape as the one in your imac.
[http://store.mcetech.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=OB-PBG4&Category_Code=STORHDOPTIBAY|Optibay]