Im using my gmail account to use ichat, but i want to know can i only add people with gmail accounts, or can i add anybody that is on ichat, no matter what email server they are using like @me.com or gmail, or aim
Google Talk uses the XMPP/Jabber protocol (and is the official standard for IM). Google also federate (unlike Facebook which uses XMPP but doesn't federate) which means you can add contacts from other servers also using XMPP (such as Livejournal and thousands of other domains).
I had hoped that when Apple changed the .Mac service into Mobile Me they would switch from the AIM protocol to XMPP on the front-end and do some black magic on the backend to let users still chat to those on the legacy AIM network. Alas that never happened.
.Mac, Mobile Me and AIM all use the same network, so if you want to chat to them you will need an AIM account. Personally, i'd just try and get them to switch to XMPP if they don't already have an account.