I introduced my 4 girls together this week all fine now and TBH no real problems!
We did neutral ground (we used my downstairs bathroom as my older girls had already been everywhere but there in the house) we had them with lots of food and no hidey's for an hour or so, then added a tunnel hidey as we didn't want them to corner the babies, then when it seemed all calm and they squabbled over space we added the pigloo, but kept VERY close to make sure the big girls (2x2 year olds) didn't corner the babies (2x6 week olds) they were all together in there from about 2:30-3pm ish until i put them in their hutch at about 9pm, then we kept our door open and if the babies squeaked we checked on them until we went to bed about midnight. We had some minor squabbles the next day, really over food bowls and Nushka (my in charge girl) just domineering the babies and bossing them a bit. Nothing HUGE really, i added another food bowl and water bottle, (they had 3 and 2 bottles and i made 4 and 3) and the pet shop told me (and i am not sure i this is right as i am NOT an expert) to dab some baby olbas oil on a tissue and then on the grease glands and they all smell similar then, i did the tiniest drop and it worked a TREAT for my girls. Not a single squabble since!
As i said i am not sure on that though