There are a few issues.
The WRX flange offset is higher, so the wheels will be pushed out further and could lead to brake fitment issues.
The mounting bolt pattern is 4 bolts vs. 3 so the upright will need some new holes machined.
The rear pilot diameter is larger, so the Fiero upright will need to be bored to accept it.
Fiero rear wheel bearing:
WRX front wheel bearing: