How do I choose an optocoupler for (19200 baud) serial communication between Arduino and RS422 transceiver?
Fronius does not suggest an optocoupler between the µC and the transceiver, but between the µC and the external network (which you do not have). But you could isolate the two components of your RS-422 interface (µC and transceiver) from each other, if you really want to.

The line driver's input current specification does not matter.

An UART synchronizes to the falling edge at the beginning of the start bit; all the following bits are then sampled in the middle of their bit time.
So your optocoupler must be fast enough that the signal level is stable in the middle of a bit, and rising and falling edges must not be delayed too much by different amounts (this is specified as the pulse width distortion).

In practice, you want a safety factor, so the specified bit rate of the optocoupler should be at least ten times the actual baud rate.

Capacitive effects in cabling depend on the actual cables; you have to measure this yourself.

Normally, you should drive optocouplers with the forward current that they are designed for (which is what the CTR is specified for).
The 6N135/6N136 variants are designed for 16 mA, the 6N138/6N139 variants for 1.6 mA/0.5 mA.

Please note that that table shows typcial times; the guaranteed times can be much worse. With the 6N138/6N138, you can improve the rise time greatly by connecting pins 5 and 7 with a resistor, but this also reduces CTR:



However, the 6N138's CTR is so large that this does not matter for most useful RBE values; 10 kΩ should be perfectly safe.

If you do not want to care about the CTR, use an optocoupler with a digital output, e.g., H11L1 or 6N137. (These have a fixed threshold current.)