AC / DC

Yes they can be partnered with the disparate technologies, and it doesn't matter which is in the lead. I can tell you that BNSF and UP did this (mostly) with impunity, even from the early days of AC-traction, and still do. I know other roads around during that time did too, like SP, CNW, and BN. I would be extremely surprised if any railroad that rostered AC power did not do this, at least on occasion.

There were advantages to NOT doing this for sure, but it depended on the railroads and their power management policies. I can tell you that for example BN kept their SD70MAC's *mostly* together from what I saw, and then eventually BNSF followed that m.o., and added the AC4400's into that pool when they arrived. And BNSF still tends to try to keep AC power together for coal trains especially.  But trust me, there are plenty of examples of this not being the case.

There are restrictions as to what the locomotives can do in these scenarios, mostly handled by the computers. This could mean de-rating horsepower on the DC units, among other things. Also I think there are more manual operating restrictions when running AC power with, say, an SD40-2, but it can still happen and does. The engineers just have to be aware of certain things they have to do when operating them together.