We mix and match all the time. We do find that covering the piglets in the "poo" of the larger pig helps. Also make sure the babies can get out of the way.
Feed we have an electric fence wire under which the smaller ones can go without touching and bigger pig(s) cant this allows them to feed separately. A good tip make sure you feed them all together perhaps giving the bigger pig some treats to keep her busy.
You would need to separate them all about 3 weeks before she is due to give birth.
Having said all the above, Hilary hates everyone, and I have had to turn somersaults to avoid her going in with another sow, especially when she is going to the boar. Last week desperate measures Hilary took herself off to the boar, who still has one of his ladies with him. Went down a couple of days after to find Hilary and the other lady sound asleep together in the sun, whilst the boar was on his own. So you just cant tell. Keep the wound spray handy and go for it.