i have an old server that was originally in a rack it is a quad CPU Compaq Proliant 6400R Rackmount Server Unit, the xeon chips are p3 550mhz.I don't think so, if they have differen't FSB's and processes support, you'll most likely confuse the heck out of the computer.
What motherboard do you have? I have two spare 800mhz P3's laying around i'm not doing anything with. If it'll support them, i'd be willing to part with them.