I'm new to muzzleloaders...  shot one a couple times long ago when I was a kid...  30 years later...  some horse trading brought me to own a Marlin inline 54 caliber...

trying to do some www searches and reading up when I can...

Prior Owner said 150 grains (3 50 grain powder pellets) of pyrodex...  Is this standard or should I experiment/ work up a load like I do for reloading modern cartridges?  I see I can get 30 or 50 grain pellets which would allow 30, 60, 80, 90, 100, 110, 120, 130, 140 or 150 grain combos...  

are nipples available to use shotgun primers instead of the little caps?  if so, which are better?

150 grs sounds like a lot of powder if it's only using a #11 percussion cap.  My TC .54 Thunderhawk recommends 120 grs max, and I get best accuracy at 110 grains with either Pyrodex or H777.  110 grs of H777 over a 435 gr Buffalo bullet has hammered many deer for me.  They don't go far after being hit.  And the thump on my end is much more than a .30-06 or 12 Ga with slugs.

I just got a Lee Improved Minnie ball mold and will be trying them out.

.54 is a little harder to find components for around here, the .50 cal's seem to be the most popular.