do different shell programs provide different programming languages to write shell scripts ??

are shell programs residing in /bin directory??

information about the shell program assigned to a user can be taken from a 'start up file' , as far as i know.. can such information be taken from a passwd file as well??
does these specific start up files (e.g .profile) affect the user's session??

Shell script is basically a set of commands of that shell. Most people fon't use for or switch in their normal commands, but it's possible. Available commands are different in different shells, so scripts should use the right ones.

Yes, shells (usually) reside in /bin. It doesn't mean that you can't have one in /usr/local/bin. You can. The thing is to modify /etc/shells so your shell is listed.

Shell can use information from every file it has access to. It's working with user's permissions, so it can access all the files the user can. It's started for user, so shell settings affect only that session (if they're not written to the configuration file). Shell is an important part of session, so yes, it affects session.