To install with vim-pathogen, this should work (taken directly from the readme, I just swapped vim-sensible with vimtex):
Now any plugins you wish to install can be extracted to a subdirectory under ~/.vim/bundle, and they will be added to the 'runtimepath'. Observe:
git clone https://github.com/lervag/vimtex.git
Now vimtex is installed. If you really want to get crazy, you could set it up as a submodule in whatever repository you keep your dot files in. I don't like to get crazy.
I further suggest you read the full readme of vim-pathogen, as well as other plugin managers that might be of interested, e.g.:
Regarding how to combine vimtex with other plugins: I don't understand the question. In principle, it should just work.