I think all cars should be deluxe size. The best part about modern crossover toys is that they can, in theory, fit in with other Transformers.
Honestly, I think Ectotron fits in well. If he's too big it's only by a touch. I didn't notice it until it was pointed out and even then I had to really compare the two to vehicle's widths to see it. If I have to go that far I think it's fine. I mean, even all deluxe cars aren't perfect in terms of scale. Especially if we aren't ditching out older TFs.
I think the only crossover that doesn't quite fit in is Party Wallop. Since he's based on a mutant turtle and a flat cartoon aesthetic whereas other crossovers are based on real vehicles with real vehicle details.
The scale thing is what put me off of Marvel Crossovers back in the day (only got a few recently). They're so big - they fit in with nothing! A good deal on ebay got my into getting a few more. Ironically that package seems to have gotten lost in the mail.