Substitutions

One may use substitutions. However, one must have a firm understanding of theory in order to know how to substitute; what chords can sub for what.

 

First of all, you wrote II-V-III-VI is a variation of II-V-I. Besides that lower case roman numerals denote minor chords, you have things a little backwards. iii-vi-ii7-V7-I is common. The iii and vi come before the ii-V, not after as you've written. But what is important is to understand how and why this works.

Despite only changing one note, those diminshed chords don't work the way you're intending. Instead of pointing up a fourth they point up a half-step. Consequently, putting those chords there has destroyed the harmony of whatever passage you're doing that in because they don't resolve. I think when Todd Coolman refers to unaltered dominants he's probably referring to the fifth of the chord, or may even be suggesting not to use tritone subs without having worked out with the rest of the group or at least the piano/guitar. Raising the root of a dominant chord by a half step to create a diminished chord does not constitute an altered dominant.