had problems with this also but i finally figured it out. i use handcent and you can attach audio files to send as mms. you can also change the size you're able to send under handcent settings too...
go into handcent, click menu, scroll to 'maximum mms size' set it to '1m' and then you can send ringtones under 1mb.
bluetooth is possible too if 2nd phone is also bluetooth capable.
to send ringtone via handcent
go into handcent
create new msg
then find your ringtone and send