ok, you CAN swap an Si cluster directly as long as u use the same sub-generation. meaning....if you have an 88-89 use the cluster from an 88-89 if you have a 90-91 use the cluster from a 90-91.
if you dont mind the hassle, you can rewire 88-89 to 90-91 , and vice versa, but you will need the harness lead from the other car, the cluster and the cluster hood, 88-89 clusters are more squareish, and 9-91 clusters are a tad more rounded.
as far as tapping an aftermarket tachometer to your wiring you will need to tap it into the blue wire with a white stripe, as i stated above.