Install Express/Standard side-by-side I've reviewed these two existing questions:


Install SQL Server 2008 Express on same machine as normal SQL Server 2008 install?
Installing SQL Server Express in parallel of Standard


...and I believe mine to not be a duplicate, as the details of my scenario differ slightly (and in an important way). If I'm mistaken please let me know and I'll try to edit my question appropriately.

Question:

I already have two named instances of SQL 2014 Standard Edition installed, but no default instance. I wish my default instance to be a SQL 2014 Express Edition installation (MSSQLSERVER).

But I see that the Express installer wants to put its files in C:\Program Files\Microsoft SQL Server\, where files and folders already exist from the earlier Standard Edition named instance installations (e.g. .\110\*, .\120\*, .\Client SDK\*, etc.).

If I proceed with the Express installation and accept the default instance root directory (again, which already exists under Standard), will I cause any problems for the currently running Standard Edition named instances?

Or instead should I indicate a new instance root directory to be created (e.g. C:\Program Files\Microsoft SQL Server Express\) for the new Express installation?

I'd prefer the former if possible, in order to keep everything consolidated, but I also don't want to cause trouble for myself if I can avoid it.
Is there a really good reason you do want to use a shared root directory? From my experience on SQL 2000 and 2005 it sometimes wasn't the best option.   

Having them separate would make it less likely that a change/patch in one might accidentally affect the other or edge cases where NTFS permissions/corruption or something aggravates another issue causing more troubleshooting or affecting both.  If there's not a strong reason to keep them shared, I typically separate due to a tiny amount of bad experiences.