Ok just took delivery of a PSA 7" 556 Marauder PA15 AR 15 pistol. After much research have decided to swap out the original upper minus the BCG and CH for a 300 Black out.
First will the original BCG and CH work in a 300 bO upper from PSA? What length would you get for a good home defense gun. What grain bullets would you use. Thanks alot.

Usually the 110 gr Barnes Tac-tx are the go to for home defense or deer hunting.
Very destructive bullets!
The bolt and charging handle can be used for 300 blk and 5.56 interchangeably.
I'd go with a 7-10" barrel for 300 blk.
I use different colored magazines for my 300 blk guns to make sure I don't accidently mix up ammo with my 5.56 guns.

I believe the optimal short barrel is around 8.5" (8.2").
Future host for a suppressor?
For supers Link
Can go longer without issues but shorter you have trade offs.

It wouldn't be difficult to just swap out the barrel and keep the same "look"

can 556 and 300 bo use the same mags

Yes, but some long sub bullets hang up on certain mags. The only difference is the barrel and muzzle device. My bedside gun is a suppressed 8" 300bo..

most current of many "300blk kaboomed my 5.56 rifle" thread

https://www.ar15.com/forums/AR-15/Blew-up-my-go-to-AR15-300-blkout/118-785620/

strongly recommend using a fundamentally different shape/material magazine for 300blk.  

different color but same shape/brand/material  is a risk

sharpie or paint marker but same shape/brand/material  is a risk

rubber band marker but same shape/brand/material is a risk

mine is a 10.5" and I like it.  many have much shorter barrels. ymmv.

a) yes the BCG/CH will work for .300BLK

b) You can reuse the magpul handguards if you use a 8" .300BLK pistol barrel, low profile gas block, pistol gas tube, and regular handguard endcap. Honestly, you can probably reuse your 5.56 gas block and pistol tube if they're not pinned.  Technically you probably need a .700-.730 diameter endcap, but .750 has minimal play and it's easier than buying a .625 and opening up the hole to .700-.730

Good price:
8" 300BLK Barrel $118
Aero Pistol Gas Tube $12
Low Pro Gas Block $18
.750 Round Handguard Cap $4

c) Pretty much any 8"+ .300blk is going to be solid for HD.

d) Just about any of the 110gr ballistic tip will work great for HD.  TAC-TX, Saber, Varmageddon, V-Max, Sig, etc...  

Avoid pretty much anything 145-155gr (ie: .308 projectiles loaded into .300blk cases) except as cheap range blasting ammo because .300BLK won't get them going fast enough to do .308-things.  They're just going to icepick through targets.

The ~200+ gr subsonics are fun, but unless you're shooting the $$ exotic controlled-expansion things they won't be too good for defense.  You're pretty much just lobbing big pistol rounds at that point that, at best, are going to tumble through a target with none of the hydrostatic effects of a 2000+fps projectile.  But they are damn quiet and, unlike a pistol or PCC, those long skinny bullets keep decent velocity for a LONG ways... If you can cope with the dope, .300blk subs are good to 200-250+ yards.

Seems everyone has covered it pretty well.  The only advantage to the long and heavy subs besides quietness is that they'll tunnel through soft body armor like a hot knife through butter, but as mentioned, unless you use the expensive all copper bullets designed to petal out at sub speeds, you're just poking clean holes through the bad guys.  You could make arguments for and against, but it's ultimately up to you.

If it matters in helping you decide:

https://www.youtube.com/watch?v=euucD2YJq4w

Solid advice.  I use 20 round p-mags in the 300, the only thing I use 20's in.  Also, I never have 5.56 and 300 guns out at the same time.

One thing the nobody seems to mention about those bullets, slow and expanding reduce penetration.  Some of those things spread out like a parachute.  

I would choose my AR45 over a 300 with sub's, or hell even my AR9 with 147's.  I don't suppress.

I will add, there is a new expanding sub on the market, but I have not seen it in gel yet.

PSA's AAC Sabre 220gr ballistic tip sub.  

It says it expands, but I've only pulled a round out of dirt where it curled a bit.