The sliders apply to everybody, so you can't give yourself an unfair advantage. Your opponent will get the same slider adjustments that you do.
Even then, when someone changes the sliders a message shows up in the league Message Board/News area.
https://www.operationsports.com/mlb-the-show-20-stream-custom-leagues-breakdown/
Inside the mode, the message board function is quite nifty as you’ll see game results, users trades, trade blocks, user messages and more. It’s a nice organizational hub. If a commissioner changes sliders or something like that, you will also be notified.