With the exception of the Coruscant WB, you can solo them all up to Quesh, if you're decently geared. Quesh can be solo'd if you're well geared, easily duo'd. Both Hoth bosses can be 4 manned easily. Belsavis only requires 6, Section X you could prolly do with less than 8 at one stack. Nightmare Pilgrim still requires 12+ afaik.