RSS feed for multiple categories?

1. I found how you can create an RSS for a specific category, but I wanted to find out if you can create an RSS for 4 or 5 categories combined.

2. If there isn’t a way to do the above, if I create a “parent” of all these categories, would the RSS work on all the “child” forums together? If so, then is there a way not to have that parent category appear on my theme’s navbar and just have the “child” forums listed as it already is?

Thanks!