Harmattan is the first Maemo that is marketed actively and available from operators in some countries. As such there is already a fair amount of software available and it's growing more every day.
Nemo can indeed run some Harmattan apps. But you cannot directly install Harmattan apps on Nemo. You would have to manually copy the files over.
There is an article on meegoexperts.com about running Harmattan stuff on Nemo on the N900.