If you feel like a category would be fun, try it out. If other people see you doing it and join in then that's how it could become a thing, and if enough people do runs of it, it may be added to the leaderboards. This is pretty much my standard response to a new category suggestion; try it out and have fun with it. I think it's important to remember you don't need a leaderboard to do runs of what you find fun 🙂

It is fairly difficult to define what constitutes "all coins" in SMW. Because of how the game tracks coins you've collected in a level, collecting one coin in one room may make one or more coins in a completely different room fail to appear; and on the other hand some coins can be collected multiple times by going into a different room first. Multi-coin blocks have a time limit rather than a coin limit, coin snakes don't really have a practical limit, and block duplication allows for getting multiple coins from a single coin block. Yoshi can swallow enemies to get coins as well. Those and many more reasons are why people haven't really run such categories in the past even though many suggestions to do so have been made.

Your suggestion, to get around that essentially by specifically listing which coins count, is workable (though impractical for anything longer than 1 world). Regarding your question, I don't know which block you are thinking of that will give a Mushroom or Coin depending on whether Mario is big; the only similar thing is that there are blocks that will give a star or a coin depending on whether Mario already has star power active, but none in world 1.

It is probably worth mentioning that one can collect all the coins in the sublevel of YI3 by doing a Yoshi jump. There may be other methods like a p-switch jump or shelljump.