If you want to do the slo mo in camera you need to plan for it and shoot it that way. Any other way is a compromise. Going from 60 to 24, is not exact so there has to be some frame blending (lose of quality). Doing slo mo in post requires plugins to help get a good result.
But will this look horrible, no, it can be done you need to be doing test of the entire workflow it see what works best for your situation and still gives you acceptable results. With production there is no, do this and you will be fine.