I wondered if it would be possible to link certain fields between entries? For instance, if I have separate entries for Google and Gmail, would it possible to change my password in one entry and have it update in the other?
No, there isn't a way to link sections between items. However, if the Login is the same, you can just enter the Gmail URL in the Login item for Google. You can add any google related service URLs in the same Login and 1Password will show it to you when you're on the sites. Here's an example: