The IIfx slots and SE/30 use the same connector, but are not the same slot. They're similar and differences are pointed out in the second link. The IIci Cache slot is an entirely different breed, hence the warning and the need for model specific adapters for the IIci compatible PowerCache cards.
At the bottom of the first link you posted, there is a different part number listed for each of the '030 PDS slot types, including different cards for the SE/30 and IIsi, whose slots are identical, but for the bus speed. This is probably just a matter of using a lower clock multiplier for the faster IIsi System clock, but it also might be do to using a very different form factor, like the two different Radius Color Pivot II cards for the SE/30/IIsi pair.
From my read of the chart, a IIfx card would be yet another model specific accelerator/part number, if one had ever been produced. Using a 25% faster CPU/CoPro on a 50% underclocked system bus extension makes no sense at all in terms of overall system performance.
The PowerCache Accelerator/Adapter model was far more efficient than producing an entirely different card for each of the ludicrous number of different implementations of the '030's Processor Bus.
