If your main tank is cycled, your problems will be mostly longer term issues.
That pleco can grow to the size of an adult forearm regardless of tank size. Platies are excellent algae eaters, much better than an adult pleco.
If you are insistent on saving babies by separating them, get ready for population issues. Along with those population issues you will get deformed and weak fry that sadly will live out a miserable existence that will painfully fail without human intervention. Even if you get a large tank, you will get population issues with this course. Get ready for massive amounts of culling or make plans to give away countless fish.
Platies are great fish and are my first tropical fish. My original platies are my favorite fish even with the more exotic looking fish in the aquarium. My original platies helped me train my angelfish to feed off my fingers.
I never separated my platy fry from my main 55 tank. With the rummy nose tetras, the near full size angelfish, and cannibal platies, hardly any fry survive. My tank is planted on the corners to give any fry a fighting chance. I didn't need to feed them extra, platies are excellent algae eaters and the babies have plenty of food until they can munch on a falling flake. The babies that do
survive until a safe size are extraordinarily strong, fast, and smart. When I used to see the angelfish try to chase a bite-sized baby it was like watching a 747 chase a jet fighter. The angelfish doesn't even try for any young platy that is brave enough to swim in the open anymore.
Now I am not suggesting you go my route, I have my issues as well. Every single fry that survived until sexual maturity are females and highly aggressive. Little punks. Just wanted to share my experience.