for the current mirror you can use a multiplicity greater than 1 for the minimum transistor, i.e. 2:2 instead of 1:1 and use for example ABAB. If you have 1:10 it is of course a bit more complicated and boring. You can always increase the minimum and put fingers between those of the other transistor.
For diffamps, yes, common centroid is the best. In case that you need very good matchin performances you should use it. However, in case offset is not a major concern, you can ignore it and use simpler simmetries. The mosfet sizing depends on all of the design specifications, gain, noise, speed, bias, and yes, offset. Remember that offset is proportional to 1 over the square root of the MOS area, fingered or not.
Your way of doing common centroid is good. You can also bring it to the extreme using something like that:
MOS 20x1 (for example) you can divide it into 4 MOS 5x1 so that u have A1B1C1D1 and A2B2C2D2. You can put it like that:
A1B1 C1D1
C2D2 A2B2
or
A1A2 C1C2
D1D2 B1B2
there are a lot of ways of combining common centroid, simmetry and interdigitation. However, in my opinion, it is not always useful. The layout should be made with care that increase with the required offset permormance. If your amplifier should have by design 10 mV offset, just do simple common centroid or simple simmetry, size properly the area, and that's enough.
Check the book "Art of Analog Layout" from Alan Hastings for details.