With regards DRAM chips:-
Yes, there are two variations of 64k X 1 bit DRAM chips. The different refresh requirements depend on the actual internal layout of the memory cell grids. The type that has a 128 cycle refresh was designed with older 7 bit refresh systems in mind, not just the Z80 CPU (although this would have been a big part of the considerations).
While talking about the Z80 and refresh, I don't know of any Z80 CPU that has 8 bit 256 cycle refresh circuitry, whether made by Zilog or a second sourced Z80 CPU. The later and more advanced Zilog CPUs may be what is being talked about, or the specialist Z80 like CPUs.
With respect to the timing, you also have to allow for the actual way the CPU accesses the memory. Not just the CPU clock rate. Then the time taken by the decoding logic and multiplexing circuitry also has to be taken into account. For the RAM used by the ULA to generate the screen, that timing has to be considered as well. DRAM that is too slow may work some of the time, but may cause strange effects, or random crashes.
With the "upper" 32k bytes of memory in a 48k machine, the 256 cycle 64k x 1 bit DRAM chips can be used, as only half of the capacity of the RAM chip is actually being used. So it does not matter that the unused memory cells are not being refreshed.
You can mix some types of DRAM chip. But it's not normally recommended. Because you need to carefully compare most of the details in the datasheets to see if they are suitable to both work in a system.
The 128k Spectrums have additional circuitry, so the DRAM arrangements are a bit different.
What chips do I use? It very much depends on what I can get hold of... If it is easier or cheaper to replace the lot, then so be it. Or if I have another that has the type of DRAM chip I want, and they are in sockets, I put the other manufacturers DRAM chips in the Spectrum that has the sockets, and rob the existing chips for use in the board that I am repairing...
For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.