The plugin has to be the same version as FM. Both 32bit or both 64bit. It is also recommend to use the same version of software. (QuickBooiks in your case). So either use all 32bit or all 64bit (does not include OS). 32bit software will run on 64bit OS. It is OK to run 32bit software on 64bit OS.
That is an interesting question
For your Accountant, it seems the Plugin is the limiting factor. FMBooks Plugin is still 32 Bit
However, the rest of you can use 64 Bit FileMaker
> For all the clients to talk to each other, through the server, do we need to all be one or the other (32, 64 bit)
I use a plugin for QuickBook Online from Productive Computing. I was told by Productive that Quicks Books only supports 32bit so for it was an easy decision.