I've seen manufacturers like hevi steel and boss make loads with different size shot in them and it just seems to me that if you have a size 3 and 5 shot in the same load your shot string would be a lot longer because one shot is going to physically be faster than the other... Am I wrong in my thinking?

They are both launched from the same cup/wad at the same speed. I think it would be minimal.

I think so too. We're talking <50 yards, it's probably not even statistically relevant at that range.

I fooled with a mix of heavy shot and steel and also the old duplex loads when they first came out. I never got past the pattern board though. I never found a load that patterns suited me enough to hunt with and gave up on it.

They leave the barrel at the same speed. What will be different is the energy they carry. Also, the rate at which they lose velocity, which is irrelevant at reasonable shooting distances.

That probably sums it up pretty good.

Ive wondered the same thing with their #3/5 blend. I know some guys in Oklahoma using them and they swear by them. But they are getting alot more mixed bags of ducks and geese compared to what I get around here.

Interesting read here mentions duplex loads and talks about stacked loads.

https://www.wildfowlmag.com/editorial/a-closer-look-at-migra-ammo/380421