Did you try something like... ""+worldIn.playerEntities.size() ?
Because a raw integer value isn't terribly useful. You'd probably really want to display something like "5 players in world" which would be worldIn.playerEntities.size()+" players in world" which wouldn't have your int->string conversion problem.
You can just use them directly from your JSON files. If you wanted to use the Iron Block texture, you would use minecraft:blocks/iron_block for the correct texture in your model or blockstate.