You can run "gulp watch" to compile the files into /static, and Hugo's live reloading will pick up the changes.
I can't find anything about slim templates. "Hugo uses Go’s html/template and text/template libraries as the basis for the templating.": https://gohugo.io/templates/introduction/
It does support partials, and the go templates are similar to liquid.
I honestly don't do much to customize Hugo, I just pick a theme, tweak the CSS, and write some blog posts. If you have a Jekyll blog with a lot of custom Ruby, then it might be difficult to switch.
These are the things that keep me with Jekyll and middleman.