For various reasons I find that a plumbing job on which I have embarked
involves using plastic (HEP) pipe for various awkward runs with copper at
either end. Am I right to assume this can be done without restriction?

In some places I am using straight metal compression couplers between copper
and plastic with an insert in the plastic pipe. In other places I have
plastic elbows with plastic on one branch and copper or chrome plated copper
on the other. Are there any gotchas I should know about?

Excuse the paranoia, but once completed most of this stuff is going to be
under the floor of a small bathroom and very inaccessible. I dread having
to rip out my carefully installed fittings as water seeps through the
ceiling below!!

Frank [the tentative plumber]