if it is correct that Maximum Liklihood classification algorithm takes into account RGB values of the bands only, then is it useless to stack other bandslike NIR SWIR etc prior to go ahead and use stacked image to retrieve landcover classes?
Maximum Likelihood Classification, in any remote sensing software, will consider all of the bands passed to the tool and not be limited to the RGB spectral space.
However, one thing to keep in mind is that Maximum Likelihood does not do very well with data in different scales so, for the best results, you want to match the bit-depth of your data. For example, if your NDVI is in a floating point (-1 to 1) and your source spectral data is 8-bit you would want to rescale NDVI to 0-255. Maximum Likelihood is also not a very good choice in algorithms when including ancillary data (eg., slope).