That's true, they would have completely different play styles. I do doubt they have a series cap. I'm just thinking in terms of voters. There are people like me whose favorite Smash franchise is Pokémon, so no matter what I was going to root for another Pokémon character. If I didn't like the idea of Sceptile so much I probably would be funneling all of my votes to Delphox instead. So in that sense they're competing. That's an interesting point, that they could have looked at Delphox in development. Is that just a theory or do you have something to back that up? I was under the impression that it was always just Greninja, though I can't find the article that gave me that impression.
It's mostly speculation on my part, but it follows a logical process:
We know how they chose Greninja. Even though roster talk for Smash 4 started a little over a year before the release of Pokémon X and Y, they decided it'd be a good idea to have a Pokémon from Gen 6 in Smash since X and Y would be a year old by the time Smash 4 was out. But since the new mons were only a ways into development themselves, all they had to work with for Greninja were (finalized) design documents, with concept art, design notes, et cetera. They liked what they saw, went forward with Greninja, and based on how popular the 'mon is, they made a good choice.
...But what
other mons would they have looked at? It wouldn't make sense for them to only have
one candidate from Gen 6. There's a pretty good chance they had other candidates as well, others that just didn't measure up as well as Greninja did. And if there
were other mons, then as a prominent starter Pokémon of the popular Fire type, the first in a long time to not become part Fighting, and with a spellcaster motif when mages are always popular, Delphox
would have been one of the other mons they looked at.
...Along with probably Chesnaught, Sylveon, and Hawlucha. Hard to say. Like I said, just a theory of mine.
But it'd still mean those characters had a foot in the door. And if the developers just happened to get interested in one of them even though they were passed up on before? Who knows? Maybe we'll see one pre-Ballot. Maybe the fact that I keep running into fringe Delphox support in places could mean something.
But enough about my character, lol. Sceptile's chances (imo) lie mostly on the Smash Ballot and on looking like a more compelling option than Ivysaur.