We’re building an internal implementation of something like Gmail and have not yet decided on a design approach. The interface will be very similar – with chat, notifications, emails, etc. The code is in place but we haven’t pulled the UI together.
It seems like TB supplies front-end devs with a framework to base UI on, however I’m not sure how “strict” the framework is. We don’t want to spend more time customizing the framework to achieve the look we need than we would just implementing our own HTML/CSS/AJAX etc.
How (easily) customizable is the grid/layout with TB?
Twitter Bootstrap is great to get started with, but it’s very easy to get stuck in the same design patterns that it encourages. Then, if you want to make it more customized, you’re overwriting their properties, and that adds a lot of code bloat.
I’d recommend a more minimal framework. You’d be writing more code, but you’d have more control over how it’s presented. If you can use SASS, I’d recommend something like bourbon and neat from thoughtbot. If you’re not using SASS, they might not work for your exact purpose, but they’ll give you an idea of what a lightweight framework could look like for you.
EDIT:
I reread your question, and it looks like the grid component is important. In that case, I’ve loved Skeleton for a good responsive and adjustable layout, and it comes with sensible defaults that are easy to override, and not a lot of code bloat.