The need to filter the lights will depend on the actual shift in color as well as your tolerance for the differences in color. Typically, the shift in color from two flash units is not so great as to be noticable. However, if you are photographing neutral colors (grey, white, or black), then it can be more apparent.
The way you gel a particular light with a particular modifier will depend both on the light and modifier.
David Hobby (a.k.a. The Strobist) has a write up for gelling various lights. You cna check out his blog here:
http://strobist.blogspot.com/2017/02/ho ... ights.html