The scale creep will mean less for some models. I doubt using the new Black Orc Blockers in on old team is going to be a problem - may even enhance them. Conversely, you may want to keep your old Goblins to emphasise their puniness.
I'm just putting the finishing touches on an Orc team I've had on the backburner for years, mostly made using 40K parts. Linemen are Ork boyz, Black Orcs are Nobz and Blitzers are a mix (mostly biker and unit boss parts). Goblins from the 40K Grot set. The thrower turned out to best be made from a WHFB Orc
I think this is a fairly common practice, from team pictures I've seen on the web.
In any case, they're already larger than the original 3rd ed Orcs and should scale fine. I'm actually thinking about buying 32mm bases to rebase them onto, but not much point unless I get the new scale board.
To be honest, the old Human models were already a bit small compared to WHFB ones, even pre-AoS, and my Dark Elf team, converted from the original 3rd ed plastic 40K Dark Eldar Warriors are VERY small next to their own cheerleaders, converted from plastic daemonnetes! But we're still talking about "within normal height variation". I guess the players of yore were all short-arses