All of ERDAS IMAGINE's indices are built on Spatial Modeler programming language (indeed, about 95% of Image Interpreter.) The Model Maker provides a graphical design tool to build spatial models in the Spatial Modeler programming language. Personally, I would open the Indices application and select "View" to see the NDVI model and modify that for my needs.
If you choose this method, be sure to remove the stretch to 8-bit option if you do not need it, and use Save As to create your new model.
Using this method, NDSII for TM of (TM3-TM5)/(TM3+TM5) and NSDI (TM2-TM5)/(TM2+TM5) should be very straight forward; even the NSDII for SPOT 4 & 5 of (B2-MIR)/(B2+MIR) should be simple as well.