You can use your gmail account to login at orkut. If you are using your other email address to login at orkut they you can link that mail adderss with yout gmail account so you can login form your gamil account.

Your gmail account is your google account where you can add other mail address so you can access all Google services.

You can use your other address too if you want mroe security as hacking gmail account is easier then yahoo mail. ( at least for me)

You can use whatever you want and what suties you. Its your world, do what you want. No one can stop you form doing the thing you like.

You can login orkut form your gamil account.

I advice to get Gmail account so you can use all sites of google including youtube, blogger etc form one account.

You can also login from your yahoo, rediff mail address. But you wont be able to use google talk anymore.

Don’t give your login detail to anyone. Keep them secure. Many people can make bad use of them.