Since the D90 does not support metering with non-CPU lenses I-TTL flash control is not supported.
If your Nissin Speedlight has Non-TTL Auto flash mode or Manual flash mode you can use either of the two modes with your D90 in Manual exposure mode.
Yes, the SB-28 is safe to use on your D90 though again you will be limited to using Non-TTL Auto mode or Manual flash mode. Film TTL flash control, D-TTL flash control, and i-TTL flash control are entirely different systems and are not compatible with each other. When I get to a computer I will give you a link for testing the sync voltage of your Speedlights.