a) The term "ex-colleague" might possibly give the impression that your referees are old work buddies who might not be totally impartial. That's all I can think. If you can find a legitimate way to make them sound less like they might be your friends, and more like they had a position of seniority within the company, then you might want to do that. For example, "ex-supervisor" would be better (if true).

b) I'm slightly confused. You say you've been asked to fill in an application form regarding a job offer. An application form usually precedes the job offer. Normally you fill in the application form, then if they like the sound of you they'll ask you to interview, and then finally they might offer you the job.

But, assuming you really have been offered the job, and it's not an "application form" in the usual sense, here are some suggestions:

Thanks for communicating your communication with me regarding your [I assume] job offer. I would like to propose that I will try to shortern shorten the my notice period and make myself available to start [I assume] during the first week of Sep'08 September 2008.

Once again, thank you for showing your interest and for contacting me. -- But this arguably doesn't sound quite right. "Thank you for your interest" would be more appropriate if they'd contacted you, say, inviting you for interview. If they've actually offered you the job then it's a little more than "interest".

It is, in my experience, perfectly normal and acceptable to use ex-supervisors/managers (whatever you want to call them) as referees on a job application form, and to describe them as such. I have no idea why your friend thinks this is a bad idea. Perhaps there are some unusual circumstances applying in your case. All I can do is suggest that you ask your friend why he advises you against this, and what he thinks you should do instead.

Don't worry at all about distrubing us, Rajay! This ended up clearing up the confusion for you. Good luck with your job.