It isn't just the exact size of Mollies, it is the fact that even a 3" fish needs more swimming room than a 10g. They are very active , messy fish. All Livebearers (Mollies, Platies and Guppies ) have Large bioloads for their size. They poop a LOT.
Also in small tanks Mollies often turn into murderous Bullies !! In 30g+ they are more peaceful, but IME I still consider them Semi Aggressive esp around food.
Mollies are Not suited to 10g tanks for long term. Even short term can end in death.
The inch per gallon rule is old and causes a lot of fish abuse. By that "rule" you could put a 10" Oscar in a 10g.
I've had tanks since I was 12. I'm a mature woman now. I've also worked in fish stores over the years.
I don't mean to sound harsh , I just really want your tank to be successful and your fish to live and THRIVE !!
If you want to keep the Platy ? I'd probably only put 2 Platies in a 10g.
I like more fish, so I use Nano fish in most of my small tanks. I don't mix big fish with them.
I stock Celestial Pearl Danios 5-7 and
Red Cherry Shrimp
Or look at Ember Tetras or Micro Rasbora species. Embers may hunt Shrimp.
Tiny Cories like C pygmaeus can work also.
I use lots of live plants.
Check your lighting and post what type or start a new thread in the FW planted tank section for help with plants/lights/Ferts.
It's so exciting to get a new tank
I hope you get to enjoy it for many years. Please read the forum and keep learning.
I learn something new everyday
Regards Tina
member San Francisco Bay Area Aquatic Plant Society
Need to join SFAS someday as well
Coursair Whippets ( my other hobby)