So I replaced peaches secret slide with my own secret level and decided to put one normal star in and one red coin star in. The normal star worked but the red coins did not count, so if anybody knows what the problem here is and how to fix it please tell me.
I see your problem. What you need for the Red Coins to count is the spot where the star will be. Go into TT64 and insert 021 Star For Red Coins (Hidden) and there you go! When you collect the 8 coins, the star will appear!