Nothing I could do would generate a "group of sources", only "of people" with those type sources.
Yes, that's the way it works with sources. There have been numerous unfulfilled requests through the years for improvements in RM's source management. A simple filter on the source list would meet your needs very nicely, but such filtering doesn't exist in RM7. It will be interesting to see what if any improvements are in RM8 in this regard.
With respect to RM's plain vanilla Search capability (not Search Everywhere), the facility has a very subtle problem. Namely (and as you noted), it only searches for people. It can't really search for dates or facts or places or sources or citations or anything like that. Rather, it searches for PEOPLE who have dates or facts or places or citations etc. It may seem like the same thing, but it really isn't. And the same basic dialog that is used for plain vanilla Search is also used for the Marking and Unmarking that is used for making groups and for color coding. So you can only make groups of PEOPLE. You can't filter on dates or facts or sources or citations or anything like that.
For the most part RM's plain vanilla search meets my needs pretty well anyway. A major exception is that it is impossible to target two different search criteria to the same fact. For example, a search for an 1850 census in Texas will match somebody who had an 1850 census in Tennessee and an 1860 census in Texas. I get around this problem in part with SQLite queries and in part by using a different user defined fact type for every census year. The census fact problem also affects People View and makes People View essentially useless for processing census data unless you use a different user defined fact type for every census year.
For facts, one thing that would help tremendously would a new main view called Event View which would work on events (AKA facts) instead of on people. The Event View would/could solve many existing searching problems, especially if it had good filtering and sorting capability. There is a wish list item for this new view, but my hopes are not high that it will ever be implemented.
RM's Search Everywhere seldom meets my needs very well. For one thing, it's way too slow. You need to be able to target it just to places or just to sources or just to whatever. You also need to be able to filter Search Everywhere by group. Indeed, in addition to not being dynamic, RM's group facility is way underutilized. There are lots of things in RM that need to be able to be filtered by group that are not filterable by group in RM7. But most importantly Find Everywhere doesn't always give me the full context of what it found. If I try to edit what I found, it's sometimes like I'm flying blind. For some types of data, Find Everywhere doesn't have this problem but the problem is severe for the citations it finds.