Yes, that's a reasonable call.
When do you roll for initiative? It's a tricky one. The slaadi have 60ft range of vision so that aren't aware of the PCs and don't formulate the intention to fling fireballs at them until the PCs, having lined up along the back wall 65ft away, start to step forwards from the portal. That's when I would determine surprise and roll for initiative. But the wizard has 120ft range of truesight, so he sees the slaadi before they start doing that. Indeed, I might even allow the
slaadi to be surprised if the PCs do it right - stand very still, communicate silently and formulate an intention to attack. In my play-through, though, they didn't do that. They didn't surprise the slaadi.
I didn't think surprising the slaadi would be quite in the spirit of this experiment