1. i think that's just the number of assets available. so you can probably add stuff until your computer can't handle it no more, lol.
2. i think it's probably the same as the first answer. just add bugs until your game lags, lol.
3. i don't think we had a height limit on the main game, so i don't think you'd have one in playgrounds.
you should be good to build what you want. some people's systems might handle it better than others, but that's their problem.