I've been using grunt task runner in all of the angular projects I was involved in.
Currently, I found a gulp-protractor-qa plugin for gulp that watches all my element selectors in the tests on the fly. It is good at what it does, but now I have to have a separate gulpfile.js config for the another build system (I cannot find an appropriate alternative among grunt plugins).
Is it okay to use both grunt and gulp build systems in a single project? What are the generally accepted actions in this case?
gulp-protractor-qa is just an example. I can imagine this would hit me again when I would need different suitable plugins in both build systems and would have to make a choice: try to sit on two chairs?
Like you said, if you don't have any other alternative with grunt, I think that the need of using both task runners is real until an equivalent solution is found or created.
You should of course take care of the possible overrides and conflicts that comes with each task you add in gulp, and to me, never mix your watchers. Always choose to use them in one or the other runner cause of the infinite loops. Even if in this case with gulp-protractor-qa you are watching files, there is no dependent task which runs and nothing is written so you should be fine.
Also, since the new dependencies will mainly be the dev ones, the performance of your deployed app will be very slightly affected or not, except if you run your tests on each deploy, that may take a little longer to install them.