If you are referring to SP_Midi, then http://www.beatnik.com/ makes a composer.
If you are referring to normal Midi files - then you are probably better off making the ringtones using a profesional music application - then you can make files for other handsets from the same master source as well.
"Does this mean that Nokia will accept normal 16 bit midi tones?
Or just the SP_Midi format or are they the same thing?"
Nokia's handsets use 4 channel MIDI files - so it will accept a normal 4 channel MIDI, but not a "normal" 8,16,40 etc MIDI file.
The advantage of starting with a decent source MIDI file is you can then downgrade it to support fewer channels easier than you could upgrade a 4 channel MIDI to 16 channel capability.