I'm so confused about how you think this would positively impact the feature of teams in Pokemmo all it does is make your team members not trust you enough because you want to hide information from them. I get it that having a smaller team looks bad but that just means you need to give them reasons to stay in the team. People don't stay in teams because they have high number counts.
We stay in our teams because of the other benefits, I.E making friends, exciting conversations, team events, and having people to enjoy our experience with. All of those things can be accomplished with a team that has 20 members or less. It sounds like your players either don't have their priorities straight or feel like they aren't gaining much from the team that other teams could provide. I fully agree with @OrangeManiac. Wishing you the best with your team endeavours nonetheless!