Bill splitting

I would have him pay X amount rent, and clearly label it "rent" on bank transfers, and issue him reciepts.
Halves in all other bills.
I'd also speak to a solicitor & have them draw up a pre-nup (or similar document) stating that if you guys split up, he is not entitled to any claim on your property.
And if he isn't willing to do that, don't let him move in.
Also you need to get your will drawn up properly, stating exactly what he is entitled to, and what your kids are entitled to.
If you don't have a will, your estate reverts to your ex-husband, if you were married previously (even if you are divorced and living with a de facto) otherwise it goes to the public trustee, who takes a massive chunk and decides the rest.

I moved into my partners home 3 years ago. I don't contribute to the mortgage or rates because it's not my house and I won't be getting it if we separate or he dies so he can deal with that by himself. Other shared bills that keep the house going we go halves in and I pay for most of the food since I don't pay the mortgage. We pay for our own cars and personal bills plus pay for whatever our own kids need out of our own money like clothes, excursions etc but still make sure it's even amongst them. Eg if I spend $400 on clothes for my kids he matches that for his. It's a bit tricky joining finances when you have school aged kids and assets but still want to keep it mostly separate, but it can be done.