You can still do that with the Codex; you'd just lose the Regiment benefits if you include an Inquisitor in the same detachment as Guard... But that's then the same as not having Regiments, just like the Index.
You still get to benefit from everything else that the Codex improved though, and some units can be taken in a detachment with Guard without losing the Regiment bonus (although they don't get it themselves).
For example, you can have an Inquisitor plus some Psykers in an HQ detachment, and then a main detachment with your tanks and Guard troops plus a unit of Scions. In this instance the HQ detachment doesn't get the Regiment bonus (although it doesn't matter as those of those models get it anyway), and the main detachment get all the benefits of the Regiment rules (except for the Scions as they are an exception, but they don't prevent the rest of the detachment benefiting).
So again, I really cannot see any real benefit to using the Index over the Codex, except for a few esoteric options (which are still valid anyway), or to use old rules that have been purposely removed.
However, as I said above, if you and your opponents are happy enough, then use whichever rules or models you like.