i think in both competitions there are little feds which don't have enough skaters to make teams.
Yes, that is true -- going back at least to the 1990s when I started following things closely.
I don't really understand how the age limits worked before 1996-97. I think the lower limits were more flexible until then. But the upper limits for juniors might have been lower in general (and definitely were for male partners) between the late 70s and early 90s.
Sometimes federations of all sizes sent the same skaters to both junior and senior events, if age eligible for both, if they were hoping for a junior world medal but also wanted a talented skater to get experience on the senior level before they were ready deal with medal expectations at that level.
Sometimes a good result at a national competition in a larger fed would earn a skater or team senior assignments one year, and then a year or two later they'd hit a slump and get passed over for senior assignments but still be in line for junior assignments.
All sorts of different situations in different countries, with different individual skaters, with different scheduling of the international junior and senior seasons over the years, etc.