Is it easy to include a CSS toolkit that’s not in the atmospherejs repos, as I really like the look of the Titon.io framework and would like to integrate it into my POC that i’m working on.
Or do I use something like Semantic UI thats already in atmospherejs and it’s just a case of simply installing it through meteor add.
Any advice would be much appreciated.
There is no reason against using titon.io components in your meteor app.
But, you need to consider a few things like load orders, javascript initialization, loading of image/font assets etc.
You may want to take a look at how other ui frameworks have been wrapped for meteor by checking out their repositories and going through their package.js files.
In fact, you yourself can wrap titon.io in a meteor package so that w eall can use it 
Besides, you can always get specific help on this very topic at https://github.com/MeteorPackaging/discussions
Thanks for your reply.
I will consider taking that route sometime within the next few weeks.