I occasionally edit for other photographers. Weddings - so lots of files. My previous workflow was to get a catalog and smart previews from the client, make the LR Classic edits and then send the catalog back. Well. I gifted myself with an iPad Pro so I can edit on the road... not knowing I just mucked up my whole workflow. I have the using the LR CC and the cloud for my personal editing figured out. Where I run into trouble is how to receive raw files from clients, edit them in LR CC and then get them back to the client in a format where they can see the edits and adjust them if needed. I hope this makes sense.
Help me... I've spent 4 hours today bouncing back and forth with 5 different LR Specialists on the help line and got no-where. I'm going to be nice and just say that I knew more about LR than they did. The whole process has me in tears, it just can't be so complicated!
You one big hurdle is that you can only 'Sync' one Lightroom-Classic catalog to the Cloud. (although you can swap catalogs)
So to edit a client's photos on the ipad you would need to- Import their Catalog to your desktop synced catalog, create collections synced to the cloud, edit in the iPad, then Export their photos as a Catalog from your desktop.
I doubt this would work with only Smart Previews- you may need the full-size originals with the Client's catalog.
If you try to sync only the client's catalog you will un-sync your own desktop catalog.
OTOH If you are both using Lightroom (the Cloud Version 5.2) then your Client should be able to Share an Album for you to Edit on your iPad as per the instructions here-