How to make fluid web site consistent

I am redesigning my fixed-width web site to be (more) responsive and ran into some problems:

For one, I really like using a fluid width layout since a lot of visitors are using my web site in (much) bigger browsers than 1024px in width (currently minimum supported browser width). It just works well for the dashboard and statistics and tables with a lot of content - so I'm very happy with this part.

On the other side though I run into problems with sub-pages that contain less or condensed information. For example, input forms don't make any sense stretched all the way from left to right. Either the text input gets to wide or the input label moves too far away.

So I could just leave those pages as they are with a 960px width which doesn't look bad on its own but kind of breaks the user experience with other pages. Especially on one sub-pages with multiple tabs it just looks weird that the content width jumps from one tab to the next.

So here are my questions:
Is there a good way to get forms into a fluid layout?
Should I just mix fluid and fixed layout?
Is there a better solution?