Yes, pritty difficult at the start but with a bit of persuasion it can work out pritty well. I played second trumpet for my local orchestra and when I was asked to move onto first I changed. It has incresed my range and can now play super C's with a nice crisp sound. Hope this helps
I started playing trumpet when I was 12 and taught myself trombone when I was 16 and have been playing both eversince. What is so nice about playing brass instruments is that once you learn how play one you can pretty much play them all. The major differences between playing a trumpet mouthpeice and a trombone mouthpiece is air (you need a whole lot more of it) and your embersher is bit bigger. It should'nt take ya to long to adjust. Trombone is a bit of a jump though, I'd suggest a baritone so you can get used to how much air you have to blow and learn what the notes sound like, that way when you do switch to trombone and have to learn slide positions you know if you're making the wrong note or not.
Good Luck
I'm sorry, I had your question all wrong! A smaller mouthpiece isnt "easier" per say. You can probably do very good on a something like a trumpet or french horn because you've been playing a tombone so you probably know how to blow air (which essential) but whats going to throw you off is your embersher size. You dont use as much of you lips as your used to but thats not that hard to learn. Its a little tricky for a while, not as complicated as going from a smaller mouthpiece to a larger one.
If you just started playing this year though I'd stick with it for a while and really practice. Trombone is bugger if ya cant blow the air right. I'm still learning how to blow air and I've been playing since I was 12, Im 20 now.