Both spoilers have 5 fixings
2 bolts and 1 nut at the tailgate lid end and 2 bolts at the other end on opposite sides of the glass
the 2 bolts and 1 nut at the tailgate lid are the same across all spoilers and all hatches, painted or unpainted, the 2 either side of the tailgate glass are different

88 tailgate with a 90 spoiler:
the 2 bolts and the nut at the boot lid fit ok, the 90 spoiler is longer so goes further up the tailgate either side of the glass.
this won't be a problem as the 90 spoiler will cover the 2 upper holes which are way off on the 90 spoiler, but you can bond the top part.

90 tailgate with an 88 spoiler:
the 2 bolts and single nut are fine again, but the 2 bolts either side of the tailgae glass are further up and so will acutally be exposed user the end of the 88 spoiler either side of the glass. you can see about 80% of the fixing collar, so you will need to address this.

in answer to your question an 88 tailgae and 88 spoiler (painted or unpainted makes no difference) fit perfectly.

cheers