Yes they are. I have a 90 cluster in my 93. The odometer in the 92-95 has 6 digets and the 90-91 only has 5. The openings are the exact same size though, so you can put your 92 speedo into the 91 cluster and still have the Gen 1 look with the extra hash marks, but still have the proper mileage and 6 digit odometer.
I did the swap so I could have a green lit cluster and have the more hash marks and no orange 55mph mark.
The 91 has a low coolant light in the same place as the 92-95 cluster. It is just larger and doesn't have the O/D off light next to it, like the Gen 2 cluster does. What I did was put my 93s low coolant light part into the 90 cluster. If you decide to swap speedos, you need to remove the part anyway, so swapping them isn't any extra work. I have a shift light mounted into the O/D off position now, but the lettering isn't there anymore, I sanded it off.