Hi, I’ve got a couple of questions that I’d appreciate help on.

All these questions relate to a RackShack, Intel Celeron 1.3 GHz, 512 MB, RH 7.3 server

1. Does anyone have any experience in keeping one of these servers stable with more than 500 domains on it, bearing in mind 90% of the domains only do about 50MB a month traffic?

2. Is there any suggestion to where I can obtain a program for my server that lets my customers open, close and change passwords to their email accounts plus most other small changes that they commonly like to have control of?

Thanks in advance.

It's difficult to answer to first question because there are many factors that influence stability.
About question #2, your solution is called "Plesk creation utils programs".