I have a home build with CR flat bar on 1 side of the long axis. You do have to keep junk larger than dust the machine creates brushed off the rail. All I have to do is run a bench brush over them prior to cutting a sheet. The only time I have ever had a problem was forgetting to dust it off after removing cut pieces & slag, dross whatever you want to call it has landed on the flat bar. Never has sparks or spray from cutting caused a problem.
I think you most definitely need more than (1) 15mm bearing cassette per side of table. For that matter I really don't think 15mm rail is robust enough for the job. Your steppers will hold your gantry square. You will most likely have some deflection or spring in your gantry even with a linear rail on each side with 2 bearings per side.
When the pinions are disengaged from the racks, it is very wiggly.
I am of the opinion for a plasma table all you need on your long axis is V rail & bearings on top & possibly a standard ball bearing or cam follower bearing underneath somewhere to prevent racking on sudden direction changes.
On a 60" gantry I would want a minimum of 20-25MM profiled linear rail for that axis.
The bearing sets from CNC router parts look like a very nice setup. I think I would want 2 of them per side.