definitely use a harness. those things are heavy! even bass clarinet is kinda uncomfortable on a neckstrap for more than short periods of time, and bari is much bigger/heavier. a harness (assuming it's a good, squishy one- i used neotech) is very comfortable, and i also like the angle they put the bari at.
i never actually tried my contra reed on bari, because i was told they were not in fact compatable... if its the only reed you have you could try it, but get your hands on some bari reeds soon! even if it fits, it will not work as well just because it was designed differently for different instruments.
watch out for getting the fingerings mixed up wiht clarinet, especially at first- when i played bari, i was doubling bari and bass clarinet for pit orchestra, so there were some occasions where i would accidentally finger for hte wrong instrument, and it would sound gross. this is pretty much only an issue for in-the-staff-G and below.
for the lowest notes, you may have problems at first keeping them from jumping up an octave, because the embochure required is rather looser compared to bass clarinet and especially to soprano. you'll get used to it.
have fun! bari is, IMHO, the coolest of the saxes
