Celica 4 pots have been fitted to AW11's before, and there's no reason they can't fit to SW20s with the right fabricated bracket, but most people don't use them because they go the Nissan 4 pots which are far more readily available and cheaper to source, and there are brackets that people make for this already (see Delta V).
The rears I've not heard of using the celica 2 pots, but i suspect again the reason is largely due to hand brake compatibility issues.