It seems the brass framed 1851 Navy's would be the weakest and the steel framed 1858 Army's would be the toughest. Is this necessarily true? Does it matter for moderate use?
True. Only to a certain point. With a .36 cal, it's a chore to fit enough powder into the chambers to stress it. .44s can handle more powder, so there are some that can be affected. I prefer Brass because of it's looks. Honestly, there isn't any real purpose to push a BP revolver with heavy loads unless you plan on hunting with it. For hunting, a Ruger Old Army is the only way to go IMNSHO, and that's a steel frame.
Is there any difference between the brass and steel 1858 Army's? What problems occur if there is a weakness issue?
Shouldn't be any difference. My 1851 Navy's frame is 100% fine. The guts of it are toast. Hands, prawls, barrel keys and springs have given out on me ages before anything else. The brass frame isn't some little fraglile thing to begin with. It's a fairly hefty chunk of brass with a good sized steel rod threaded into it for the cylinder to ride on.
Other than possibly durability, are there any advantages of one type over another?
Yes. BP is extremely prone to causing rust to form on steel. Brass frames are more foregiving in that respect. Then there's appearance. I like the looks of brass more than anything except a deep blue finish. Combine a brass fram with a deep blue finish elsewhere, and hatchi mama that's one good looker! Call me strange for it but i like to take the Navy up to a bright polish, and watch it turn mello over a few weeks. I really can't decide if I like it "all dressed up" or "looking like it just rolled out of bed".
.36 or .44?
Your choice. I like .36 for no particular reason. Next one I buy though is going to be .44. @the low prices, and lack of crap to go through buying one there isn't much to prevent getting and ejoying both. Might as well ask about fluted or unfluted cylinders
Could Cast .357 bullets (made for cartridges) be used in place of round balls in the .36 models?
No. The results are pretty bad. For BP, with it's slow lazy twist rate geared towards a round ball of relativly low weight, it doesn't work out all that great. Finding cast lead bullets soft enough to use in a BP can be as expensive and difficult as finding the right thing. Trust me here, you aren't going to shoot the volume to notice a difference. Shooting a ML revolver is a time consuming process. I doubt if most people could fire 10% of the volume of cartridge ammo in an afternoon.
Can pyrodex pellets be used with them?
Sure can. I like the "devil's breath" of real BP myself over pyrodex. BP is supposed to be a lot of smoke and nasty stench. YMMV.
ARE THEY A LOT OF FUN?
YES!!!!
(added for safety even though it wasn't asked, and I'm not implying anything,,,just me being anal)
Black powder arms aren't toys. They are quite lethal. Many of the old timers preferred a C&B revolver to a cartridge arm well into the beginning of the 20th century because they just plain "hit harder" than anything else. Don't get lulled into feeling that just because it's an ancient design it won't drop some as quick as a bad habit. Black Powder comes with it's own drawbacks also. I read the following sentance in a gun rag somewhere "The burning rate of BP is the same, whether it's confined or open, unlike smokless propellant. If you don't know why that makes a difference, find out!" I didn't know, so I did find out,,, I feel that that simple statement is the most consice way to put it and is strong enough to convey the message. Muzzle loading revolvers can also "chain fire" under the right circumstances. A chain fire is when all the chambers go off at once. It happened to a friend of mine when he was using my Colt Pocket Police .36. Scared him witless. Grease the fronts of the chambers with Crisco of T/C bullet lube to help prevent it. Some believe a chain fire can be caused by other means,,I'll leave it to them to explaine it. Just cause i never had one with greased cylinders isn't proof positive my viewpoint is the one and only.