If you're going to mix in solvents like sulfite to soften the grain then it's not Rodinal any more and you might as well use a solvent developer.
Softening the grain with sulfite is one way to make an image smoother, but the grain won't get smaller. In fact, you can make resolution worse because the silver-solvent effects act a bit like a low-pass filter, i.e. it spreads things out a bit. D76 stock for example makes things very mushy, especially compared to Rodinal. Totally different look.