You may try the following script:
Fillet curve with preview (for some reason it does not work when using an arc and a straight line together): FilletCrvTest.py (13.0 KB) FilletCurves.rhp (21.5 KB)
Also, this script is good for adding fillets to surfaces:
Fillet surface corner: FIlletSrfCorner.py (9.0 KB)
More useful scripts:
A simple tutorial how to use scripts: