Ok natural liners i think carmen just means using the nappies unlined( hempstraight on bubs skin ) which tbh i have rarely bothered with liners and all my nappies are fine.
calendula more than likely best to ask at a health food store
longies and soakers are knitted covers. longies being like long pants, soakers look more like a pull up cover.
action plan - i would try unlined nappies with the covers you have - that will determine if its the covers or not cos if it is then the rash will still possibly be there. apply calendula cream too
if it goes away then it was the fleece inners if it stays then you may need to look at covers
HTH