It's not been 100% confirmed yet that Crewe guards will be learning MKIVs AFAIK but it seems the most likely scenario alright. Crewe guards are the only ones to sign Crewe - Manchester and they work the majority of those services between Crewe and Cardiff as well, so even before you consider the stabling situation it seems a sensible move.
On the drivers side at present one of the three links at Crewe signs them, and the second is due to learn them to support these extra services.
Current plan is all guards on the Mainline side of Cardiff will be signing MKIVs - at present it's just one link that signs them. On the drivers side most drivers at Cardiff in the links that sign the Marches will be signing them but I don't think it's all (again at present only one link signs them).
The depot has only just opened. Since the then ATW lost the work it was covered by the senior links at Manchester Piccadilly and by all drivers at Liverpool (with Liverpool also working the Wrexham services). Since Holyhead opened Manchester have lost their competency by Liverpool have held on to some of the work for now. On the guards side AFAIK the much longer standing depot for that grade has long since had sole responsibility for all services West of Chester.
By a happy coincidence I happen to be the author of a thread that lists virtually every passenger TOC driver depot in the country and the routes and traction they sign, which can be found
here.