Can you use diuretics in renal failure to treat CCF?

There is a well-entrenched feeling that diuretics forcing the kidneys to work may have some sparing effect, but actual evidence is somewhat shaky. At least some diuretics have not just diuretic but also vasodilating effects.
ACE inhibitors are known to cause a bump in the BUN, but they're also known to be renal-sparing. There are some circumstances in which it's prudent to stop them, but overall they seem to do a lot more good than harm. Those whose renal failure is severe enough to require hemodialysis are almost universally taking ACE inhibitors, since some of the adverse effects can no longer be an issue.
As with most things in medicine, absolute answers are likely to be wrong, and what seems certain today is likely to be false when further evidence comes to light. Doctors know that when they finish their training program and start practice, their education has really only just begun.
You've wandered into areas where the knowledge base is rapidly developing, judgment is needed, and you'll probably have to narrow your thinking to subgroups and keep your eye on the latest literature for continuing developments.