Awesome work, dude. This should go down in the fandom's history as the first awesome Mega Swampert fanart! And I wouldn't worry about the tail. I think it makes more sense compositionally for it to look like that.
Haha yeah! Mega Swampert's my favourite of the three but I agree that Sceptile's looking pretty cool too, I love all the red accents on it :> Blaziken was my favourite of the three's normal forms, but I personally didn't like its mega form very much P: