Yes it is fine and one baby starts to grow it will become very uncomfortable
Its never a good idea in my opinion but if been told sleeping on your left side gives more room for growing
I'm 24 weeks and still occasionally tummy sleeping. It's fine. It won't hurt baby. It will make you more uncomfortabke than them.
I would start working on side sleeping now since you wont be able to sleep on your stomach later on because It will be uncomfortable! I cant wait to sleep on my stomach again. But if you are nursing i found i wasnt able to sleep on my stomach for a month or so after birth!