I don’t know what you’re smoking because this is blatantly wrong. Cycles uploads the entire render kernel onto the GPU and renders from there, there’s next to no interaction between the CPU and GPU after Cycles starts rendering. This is why people can make PCs with a lightweight CPU and 8 or more GPUs using PCIE splitters and the whole thing works fine.
Again, completely wrong. You can use any combination of GPU’s for rendering as long as they all use the same API (OpenCL/CUDA/Optix). That is, all AMD using OpenCL, all Nvidia using Cuda, or all Nivida 2000 series using Optix. You can also use a non-compatible GPU to drive the screen while the others do the rendering (Ie. using an AMD gpu to drive the monitor while using 2000 series Nvidia RTX to render with Optix).
Anyways, to answer the OP’s questions…
1080 Ti are still really good GPUs. 11GB of memory is nothing to sneeze at. One downside of replacing or combining them with 2070s is that the 2070 has only 8GB of RAM, meaning you’ll be limited sooner on scene size/complexity. I don’t think Blender has support for NV-Link so you’re going to be limited to 8GB no matter how many 2070s you have.
An upside of using 2000 series GPUs is you’ll be able to take advantage of the experimental Optix API introduced with 2.81 or 2.82 which has some serious performance gains over CUDA, with some caveats. For example, you can’t use simultaneous GPU/CPU rendering, and you can’t use out-of-system memory for scenes bigger than your VRAM.
As I mentioned above, you can run almost any combination of GPU. Every Nvidia desktop GPU and most laptop GPUs since the 600 series meet the CUDA requirement, while 2000 series RTX cards can run both Optix and Cuda. As long as your case has the space and your motherboard/CPU has enough PCI lanes you can throw as many GPUs as you want at Blender.
If I was in your shoes, I’d get the 2080 Ti and try and squish it in with both 1080 Tis to keep that 11GB memory advantage, and then maybe keep an eye out for other good deals on 2080 Tis to replace the 1080s so I could go for Optix.