Honestly I don't know why you would jump into stripping them with no evidence that they needed it. Give them a normal nappy wash (less than normal detergent and a couple of extra rinses) at 60 degrees. If the water seems really bubbly or they smell musty once they've been washed and you warm them slightly (put them in the warm sun or tumble dry just inserts - no PUL or outers - on low heat for a couple of minutes), then you should strip them but most nappies won't need it. Stripping doesn't get rid of stains - if you're doing it to remove stains, just wash them and leave them in the sun or use a little stain remover sparingly and rinse it out thoroughly.
The wash at 60 degrees will kill any bacteria in the nappies... if you're worried, use a little cloth safe sanitiser but I wouldn't bother. If they smell of detergent when you get them and it doesn't wash out straight away then I'd strip them but otherwise I'd just wash normally first. No point washing time, effort, water, electricity etc.
You can wash old and new together but I wouldn't wash all of the new ones, especially as you don't think they'll fit from birth. Wash one or two, use your newborn nappies first, try out the new ones as LO grows into them then wash the rest when you know they work for you or return/resell as new if they don't work.