Without having more details on the service, a flanged flex/braided hose could be used.
If you need to know what should be used ... apart from it being always a personal choice, provide more details.
Service = biogaz with a vacuum pressure of -1 psi.
32% Methane.
I was thinking about cast ductile iron with a corrosion Protective Epoxy coating (see attached file).
SBR (Buna S) Rubber or maybe update it to Nitrile. ?
I don't know the answer to your question, but I noticed the coupling you provided detail of appears to be an unrestrained, "stuffing-box" or mechanical type coupling. Before you apply same, I suggest you check the manufacturer of the coupling and pipes for their opinion of this coupling application.
Just google "PE to steel transition fittings".... and talk to a decent looking supplier about your particular requirements. From the think you attached it doesn't look like you need much flexibility and you need to be careful about using seals to provide the flexibility. A lot of the mechanical joints like that ave very low or no axial strength and can easily pull out if subject to such forces.
BTW PE doesn't normally come in inch nominal size, but metric nominal OD such as 200, 225, 250, 280, 315 etc which often falls between standard steel nominal sizes. You need to specify which PE size you mean as "8 inch" could be any one of two or three sizes (200 or 225).
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
