Is it possible to run a shared php-pool while keeping security?

[zsero]

I'm trying to configure my first VPS using nginx and php-fpm. I've set it up
and everything is working well, but I've realized that for every user
created there is a separate php-fpm pool what takes about 30 MB of memory
even when idling. It wouldn't be a big problem on a huge machine, but I'm
trying to do this in a 128 MB lowendbox, so it's not really nice.

I've asked in a thread on lowendbox, and received a lot of help in this
thread, but I'm still confused:
http://www.lowendtalk.com/discussion/comment/123647#Comment_123647 

Can you tell me some general guidelines how would it be possible to both
have a secure setup and one that runs on low memory servers? Or it's not
possible? What I'd like to do is very simple:
1. Have users separated, having separate accounts, scp acces, etc.
2. Only have a single php-pool for all of them, not for each one of them.

Is it possible to do it in a secure way? I mean I figured out that I can run
the shared pool with a group-user, but then 'alice' can write a php script
to list 'bob's files. Are there any better solution? How is it done in
proper shared hosting, do they physically reserve at 64 MB RAM for each
user?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,230730,230730#msg-230730