Whether or not a given expansion joint is suitable for slurry service is for the manufacturer to tell you. I would be disinclined to use them as I expect any solids that can get trapped in them ruins their performance. Whether or not there are rubber bellows that can be used instead, I can't say. The pressure isn't too unlike industrial tires for vehicles, but I can't tell you what the life expectancy of rubber in your service will be, especially if you're saying it's high velocity.

With regards for springs on vessels, I would recommend a write-up on how these springs should be treated for and after installation. Something along the lines of:

"Springs should not be released until after piping is installed."
"Springs should be brought back to their set points and locked prior to breaking of flanges."

Vessels on springs have historically been a pain to deal with because it's not documented what the analyst expects them to be used for.

I would anticipate that you could eliminate the vessel springs altogether if you have a sufficiently flexible expansion joint between the vessel and pump top nozzle. If so, I would anticipate you'd support the vessels at the same elevation if possible to avoid nozzle stresses, even if it means holding the smaller vessel up by its nozzle. But this is completely dependent on design temperatures and weights. It's generally worthwhile to beef up one nozzle to handle a bit worse sustained loads and no thermal loads than it is to beef up two nozzles, add 4 springs, expansion joint between equipment, etc.

But if they already ordered the equipment, then you might already be backed into that corner.