I always pre wash all the new nappies when I get them ( a bit of a nappy habit here ) I use either soap nuts, or just a washing liquid, like fairy, or surf small and mighty. Don't use the persil small and mighty with comfort fresh... I used it, thinking it really did just have the fragrance of comfort, but it made all our nappies leak terribly. It must have bit of softner in it!
Currently we're using tropical surf small and mighty, as they were still smelling of wee after drying. which never used to happen.
I do a rinse cycle on the machine first, after just dry pailing the nappies until ready for the wash.
So, in short anything is fine really, so long as you don't use fabric softener!
When using soap nuts, which incidentaly are fantastic for all wshing, I put quite a few drops of lavender esseintial oils in the softener compartment, they come out smelling lovely and fresh, ad of lavender!
again, short list here the;
dry pail
cold rinse in machine,
add soap nuts and essential oil,
or, put in powder or liquid wash. also can add lavender oil if you like
HTH?
sorry a bit rambling!