Will Separate WP installations create problems?

I’m asking this because my site has separate WP installations to have specific facilities which I can’t have on a single WP theme or installations.

Heres the story.

ABC.com is my main site
I installed another WP on ABC.com/xyz and made it the forum by uploading a forum theme because my main theme doesn’t come with a forum (I don’t want to use forum plugins).

Now, I installed another WP on ABC.com/PQR and made it a membership site.

So my questions are.
1. What should I do in order to have same user accounts and settings for all the installations?
2. When a user sign ups for one section (WP installation) It should automatically sign him up for other WP installations.
3. Will it create any future problems or stability issues if I have my site broken down into different WP installations?
4. Will Multisite would be a better option for my case.

Answers are appreciated.