If you are using a flat formation or formation with no wingers, you should play very fluid.
If you are using an asymmetric formation, you can play all fluidites.
If the two above is not the case, and your formation is using an AMC, you should play fluid or very fluid.
In all other cases you can use all fluidities except highly structured.
Highly structured uses these roles and duties:
GKd, LDd, LFBd, SWEd, DMd, WBd, CMs, WMs, Ws, AMs, AFa
Structured uses these roles and duties:
GKd, LDd, FBs, SWEd, DMs, WBs, CMa, WMa, Wa, AMa, AFa
Flexible uses these roles and duties:
SKd, CDd, FBs, LIBs, DMs, WBs, CMa, WMa, Wa, AMa, DLFs
Fluid uses these roles and duties:
SKs, CDd, FBs, LIBs, DMs, Wbs, CMa/d, WMa/d, Wa, AMa, DLFs
Very Fluid uses these roles and duties:
SKa, BPDd, FBa, LIBa, REGs, WBa, RPMs, WPMs/a, TREa, RAUa, DFd
This is basically how I setup my tactics. I consider this approach foolproof, and it has not let me down yet. It does not mean this is the only correct way to play the game, this is just what works for me.