A point of clarification: troops can be reused across as many teams as you have unlocked. They’re not “assigned to” a given team, and unavailable in others. So if I have 1, say, Drake Rider, I can use it in team #1 and team #2 if I wish. What I can’t do is assign two in one team, since I have only one copy.
That said, @TaliaParks is correct; if you have a team of 4 Rock Worms, and you lose 5 of the 6 you had to ascension, the team slots 2-4 will be emptied automatically (and won’t be refilled when you get more Worms).