Floors 1-100 you can do a matched party, and PUG with randoms.

Floors 101-200 have to be done with a fixed party that has cleared floors 51-100 with zero party deaths (no wipes, individual deaths don't count towards Party KO).

So, basically, if you want to do floors 101-200, you need a psuedo-mini static (be it with PF members who can agree to see the run to the very end, even if it can't all be done within a day, FC mates, or members of your current static, if you have one) to run floors 51-100 with, and then you can begin to progress through 101-200.