There are a few things that I would mention to look for while choosing substitutions though.
The idea of substitution, at least in the traditional sense, is that different chords fulfill a function within your composition and that you can choose other chords that will fulfill the same or a similar function.
Starting with the tonic, Progression 1 may go anywhere from there, so we choose to use the "ii - V7 - I" progression for the next part of Progression 1, as well as for the final part of Progression 1 (since the previous part ended with the tonic too, we could very well use any valid progression after that, so we chose to use "ii - V7 - I" again). Tritone substitution dictates that a dominant chord (like V7 in Progression 1) may be replaced by the dominant chord constructed starting a tritone above (by ♭II7 in our case). Tonic substitution dictates that chords iii and vi may replace chord I. For instance, are the following progressions valid? (Progression 2) I - ii - ♭II7 - vi - ii - V7 - I (Progression 3) I - ii - ♭II7 - iii - ii - V7 - iii (Progression 4) iii - ii - ♭II7 - I - ii - V7 - vi (Progression 5) vi - ii - V7 - I - ii - ♭II7 - I First off, every progression is valid.
Whether it sounds good, makes sense harmonically, or is what you want is entirely different.
The common tones between two chords may not be enough to justify a substitution.
For instance, if you were to choose to substitute iii for I, then you will want to pay attention to whether or not the tonic is in the melody since that is the one note that is not shared between the two chords.
Theory is basically only rules when you are trying to authentically imitate a specific style, so you will actually find that there are different "rules" in this regard for different genres.With these notes in common, the two chords can fulfill the same function but there are differences between these chords, one of which being texture.When choosing a substitution, it's important to pay attention to what the melody and other important parts of the arrangement are doing.The effect I just talked about while not working for functional harmony may fit in a piece you are composing.You need to be aware in general of the effects each of the substitutions have on your line and when you can do it and when you shouldn't. They all work, but some could do with careful voicing along the way.