Tacoma,
Yes, by substituting you are definitely going into never-never land. A good read is the Hodgdon Shotshell Data Manual, specifically an article called Component Selection & Performance by Don Zutz.
One example shows a 1-1/8 oz trap load where they substituted hulls and kept all else the same. The same amount of shot, the same wad, the same primer and powder...only the hull was changed. Pressure went from 10,400 PSI to 14,800PSI with this one change.
A second example with essentially the same components and only a primer change gave pressures from 8,400 PSI to 11,200 PSI.
A third example where all remained the same except for a wad change saw pressures ranging from 8,100 PSI to 10,800 PSI.
The last example was crimp depth. With all other things unchanged except crimp depth, pressures went from 9,300 PSI to 13,100 PSI with crimp depths ranging from 0.030" to 0.090".
So to answer your question....YES you are definitely doing something dangerous.
Get yourself one or two good reloading manuals for shotgun and follow the recipe exactly, right down to the brand of primer and you'll be able to count to 10 on your fingers and thumbs for the rest of your life.
Take care,
Ka6otm