But I think its not a good idea to use different std.cells in the design. Because, if most of your blocks are 2v std.cells and one of the module has 5v std.cells, How can you make sure that the right amount of voltage is passing through. Maybe you might not get the correct functionality on silicon. You need to build level shifters to convert 2v to 5v and feed the 5v module and its output should be down converted from 5v to 2v again. Seems to be a hectic task.