For example, let's say you have a stealth suit and then get into the Power Armor, will you still be wearing the stealth suit inside the Power Armor?
I'd suspect it is highly likely, after all, I can't see you taking off your light or medium armor just to get inside the Power Armor.
And it is known that Power Armor suits were manufactured to fit around other armors:
http://fallout.wikia.com/wiki/Recon_armor_(Fallout:_New_Vegas)
In this case, this armor is made specifically to function in tandem with the Power Armor, providing added protection for the operator.