The manual is quite correct in theory - and seems quite clear to mem
However it never seems a good idea to mix different speeds of RAM in the same system. They will all have to run at PC2700 speed for a start. And PC32oo RAM, being a different specification, with different sinal timings, may not run happily at PC2700 sppeds.
Then again it might - so much depends on the motherboard and RAM modules themselves, that it's impossible to be categoric.
The worst thing is that it appears to run fine, but under heavy loads gives intermittent errors, which are simply the very worst kind, and virtually impossible to troubleshoot easily.
Long ago I decided I would never build (for myself, or to sell) systems with mixed RAM specs, ever. It's a policy that has served me excellently.