The Epic P&S rule book is the same as the main version in terms of the core rules. The difference comes in that the Epic version gives some guidance on how to use the Warlord Epic figures to form units, and the army lists are focused on only Thirty Years War and British Civil Wars (as these are the only Epic figures released).
For your 6mm collection on 40 mm frontages I would suggest you buy the regular rule book as this has lists for Ottomans, Poles and Imperials in it. When you form your units then you just need to aim to have (roughly) the same frontage for units on both sides. e.g. use 3 x 40mm bases to form a regular sized unit, 4 for a large and 2 for a small.
You may also chose to half the move / shooting distances etc. or use cm rather than inches.
Both sets of P&S rules are really scale agnostic.