Thank you very much. It helped me quite a bit
Cheers.
There are already several such tools to generate unique username. Some of them are:
1 Jimpix
2 Best Username Generator
3 Speedy Password (Personal Favourite)
4 SpinXO
5 Screen Name Generator
6 Rum & Monkey
There are other great techwhoop.com/random-username-generator Random Username Generator websites too but I would suggest you to read more about them before deciding which one would be best for you.
Hope it helps you.