Plastic and compression are fine… sort of..
As others have already stated, you must use the correct insert (ie the one for the pipe brand, and if using Speedfit, the TSM, not superseal). Do not swap inserts between brands, outside diameter is standardised but inside/wall thickness is not. A sloppy insert is not effective.
Do not use any sealants or PTFE tape with compression fittings when making the joint. If a fitting weeps after making it, then you can add a wrap of ptfe around the olive or a smear of LSX but generally a weeping fitting means it’s not tightened enough and adding sealant material at the point of making the joint can hide an inherent mechanical weakness.
Critically, tighten the joint properly. Most compression joints on copper are under tightened by manufacturer’s own instructions (where supplied), and very few manufacturers provide specific instructions for forming joints onto plastic (which is viscoelestic and has significant wall thickness so has the potential to shift over time). I’d have to check my records but I think only IBP/Conex supply specific instruction for plastic pipe. Likewise, plastic pipe manufacturers don’t provide specific advice because they don’t make the fittings. I would always recommend tightening a compression joint onto plastic pipe significantly tighter than an equivalent joint onto copper. Hand tight plus 1.5 turns would be a good starting point.
Check the instructions for the heater about the use of plastic into the box. You can’t do this with a boiler’s flow and return but I suspect you’ll be ok on a instant hot water heater. I think I saw an earlier post suggesting a service valve should be added before the unit; not saying this is wrong but just check instructions as some units require a particular length of pipe to allow hot water to expand into.
For a bit of context, I’m not a plumber; I’m a forensic investigator that has somehow specialised in plumbing failures. Very good at pointing out how not to do it but can’t solder for dung!
