I recently inherited a 1858 New Army 44 cal cap and ball bp kit gun. It is completed and was previously fired. It is the brass framed Pietta 8" model.

I read that max load is 15 gr BP.  Wow that seems light. The gun included his powder measure and it throws 25gr of FFFG confirmed on a beam scale.

I'm not sure a ball will seat that deep without wads at 15gr.

Any suggestions to get this up and running again?  

Buffer with cream of wheat or cornmeal? Bottom the ram and not worry??

Those brass frames stretch under stiff loads. That is probably where the low grain load came from. The amount of powder is not as important as having no air gaps, so if you can seat the ball down on the powder you should be good. Cream of Wheat, corn meal, etc. all work and make no difference. A lot is made of 'flashover' causing the other cylinders to go off, but if you think about it, there is no powder exposed in the front of a loaded cylinder, so I doubt packing crisco or bullet grease above the ball makes a difference, it may help with fouling, but I doubt it helps much . But if you do not have caps, or they are not sealing well on the nipples then you can flashover at the rear. Make sure you do not fire with no caps on the loaded cylinders.
Ballistol and water mixed make a good BP cleaner. Make sure you clean as soon after shooting as possible.

Thanks for the info.

I can wad to ensure no air gap occurs,  I'm just in awe of 15g being a max load....  I dont want a stuck ball and 400-500 fps is why bother territory.  I guess I need to load up and see what happens. If its too slow bump it up to no more than 20g and mic the frame length and log it over time.

I highly doubt that 15 grains wouldn't push the ball out the barrel, but I understand.  
When I shot SASS with BP .45 Colt, I was running 37 grains of FFFG, they boomed!, but I was shooting ruger's so I had no worries.
And it is an 1858, so it will handle larger loads better than an open frame Colt clone as far as frame stretch.
If you are concerned about a bullet obstructing, then start out shooting slow and making sure you see impact before the next shot.  
Good Luck and have Fun!