Chimchar didn't evolve until "Evolving Strategies" which was after the 7th gym and he still evolved before the league and ended up being Ash's Powerhouse Starter of Sinnoh.
The big difference though: There was a year of development going on between the seventh gym, and the eight gym in Sinnoh. Seeing how we would've had 5 gyms in one year if it weren't for reschedualing of some of the episodes. I don't see there being such a thing happening for Ash in Unova, there's simply no way they're going to stretch a gap that long..
There are a quite a few moments in which pokemon can evolve, they are:
- Driftveil Gym Battle: Confirmed Boldore
- In between Driftveil and Mistralton: Leavanny
- Mistralton Gym Battle: Unfezant
[]^ Two in twenty odd episodes, counting starts from Boldore?
- Mistralton City:
- In between Mistralton and Icirrus: Gigalith
- Icirrus City (including Dragonspiral Tower): Dewott
- Icirrus Gym Battle: Pignite
[]^ Three evolutions in thirty something episodes?
- In between Icirrus and Opelucid: Scrafty
[]^ One in 10'ish episodes?
- Opelucid Gym Battle:
- Opelucid City:
- In between Opelucid and League: Samurott
[]^ One in fifteen episodes?
- League:
- Post-League Filler:
Though this might seem like a lot, we could expect to see this rundown being dun to the league in little over 50/60 episodes. If we want:
- Unfezant
- Samurott
- Pignite
- Servine
- Scrafty
- Leavanny
- Seismitoad
- Gigalith
Then that's NINE evolutions.. That's an evolution almost every fifth episode, which is NEVER going to happen.. And then we're also forgetting about Iris as well, which might add two evolutions if Axew is to fully evolve.. Considering we're supposed to be expecting Unfezant, Leavanny, and Gigalith from Ash, how much else should we expect?
If we expect the following, with Samurott happening after Opelucid City:
- Unfezant
- Samurott
- Pignite
- Snivy
- Scrafty
- Leavanny
- Palpitoad
- Gigalith
If we expect this, then there would be 6 evolutions in 50/60 episodes, which is doable. One in about ten episodes is reasonable.
And yes, I think it's only around 75 episodes until the league, and that is a broad estimate..
Ash by the time he reaches Opelucid City:
- Pikachu, Unfezant, Dewott, Pignite, Snivy, Scrafty, Leavanny, Palpitoad, Gigalith.
Thta happens to be my prediction as you already know, but you are free to talk about it.
I don't think you're far off, well, aside from Sigilyph that is.. I see it having zero chances of joining Ash's team.. As we already have a team member that barely looks like anything animate: Boldore.