Why does saving them as idml files, which was created by the InDesign team specifically for your situation suck?
If you were to use idml you would be happier.
First you say you opened them and then you say they won't open.
I suggest a deep breath or two and then a real explanation of what your issues are.
Perhaps you are just unaware than INDD files can never go back a version. And IDML is uncertain and not guaranteed consistent. To collaborate in both directions there is only one good choice: to use the same version. No, 5.5 is no longer sold.
Test Screen Name,
While I often suggest that people use the same version, this situation calls for idml.
Telling someone who hasn't tried idml that it's not good, is not a solution.
I took the post as IDML files suck, because I've tried them and get issues. But maybe your reading us right.
I know there are some bugs with IDML, but usually IDML files should at least open in all versions of InDesign from CS4, CS5, CS5.5, CS6 and up to the latest CC 2017. If not, something went wrong during export or during import and maybe the file went corrupt.
What some people are missing trying an IDML workflow is that the involved inDesign versions should be at the highest bug-fix level possible. For InDesign CS5.5 this is version 7.5.3, for CS6 this is 8.1.0 (wheras I see, that only 8.0.2 can be downloaded for Windows users), on Mac OSX, InDesign 8.1.0 can be installed from 8.0.2 through InDesign's own update mechanism.
So I would check first, if CS5.5 is fixed to 7.5.3 and CS6 is fixed to 8.0.2 or 8.1.0.
To do that go to menu "About InDesign", hold the CMD key when on a Mac ( maybe Ctrl on Windows? ) and open the window to get more information about InDesign and ( if opened ) a document. The exact version number is available from that.
I know a bit how you feel.
Just today I created an InDesign document that couldn't even be opened in the version I saved it.
Maybe my fault, that I wasn't patient enough until loading completes. But It did not finish loading in 30 minutes.
So I quit InDesign prematurely and tried to load the file again. To no avail.
What is the cause of the problem?
An applied paragraph style using a simple GREP Style that made InDesign very, very slow.
Now I fear, that I lost an hour of work (one hour is the frequency Timemachine on Mac OSX is doing backups for me). And I hate myself that I did not save the document to IDML before closing it the last time. That would have given me the chance to edit out the GREP Style and hopefully to open that document again in a couple of minutes.