Detect user activation and username chars

Hello,

In order to avoid any source change becouse of possible update, I want to make extension(s) for my problems.

First of mine problems is to detect user activation (via email and with wp-activate.php file)
I know how to edit file and make what I want, but I want use plugin for it. I have plugin written by myself which works what I want, but just I need regular hook for user activate detect.

I’m using WordPress single page (without multisite) with Buddypress installed.
I’ve tried using some hooks but it offen cause plugin to start where I don’t want it. In other words, it starts it everywhere.

So, by checking wp-activate.php file I notified this line:

define( ‘WP_INSTALLING’, true );

So, I’m interest if is it possible to start plugin on user activation (via email)?

Second part of my problem is about username chars. I’ve done some research on plugins site but didn’t found what I wanted.

I need plugin, (again, to avoid any src change), which will add ability to use list of chars I want.

So, let’s say, I want to use:

[A-z], [0-9], [, ], {, }, -, _, |, \, ^

Thanks.