Perhaps some VL units for Shtil could be dropped in to replace SA-N-1, remove and replace the 2 Peel group radars with 2×2 or 2×3 Orekh. Not sure that a Fregat radar is required, but if needed this could be mounted on one of the 2 main radar masts.

Fregat is surv/tracking radar for shtil which requires 3D radar for that purpose and dont think Star can be integrated into shtil. As for Fregat its quite large radar so i dont think it can be fitted in the masts without some structure changes, even if it can be it would quite a waste of money replacing Star or Lw-08 with fregat.
Anyway as you said its waste of money fiting in VL-shtil onto those vessels, barak alone is capable enough to provide the air defense. if IN orders three more Talwars i am betting that they will replace the first three rajputs.