On Side Saddles
The answer to your question: It depends on what kind of forend you have.
On the stock 870 express, with the wood furniture, the largest that will fit on there is I believe a 4 round side saddle. If you get a different forend, you can fit the 6, and maybe even 8 round side saddle on there.
My brain drippings You might want to consider a Velcro set up.
I have no first hand experiance with the side saddles, but I've heard complaints that the hard plastic ones can let the shells vibrate loose. I haven't talked to that many people with the Velcro rigs, maybe 5-7, but they all loved them. Personally, I like the idea of being able to do a reload, and then just throw on a whole new side saddle instead of reloading from a pocket.
Velcro Side Saddles:
http://www.aresgear.com/index.php?main_page=index&cPath=34&zenid=931kl436g0psl6fnaeq3ukjkh4
On Pistol Grips
The Answer to your question: Again, it depends, if your going with one of the vertical pistol grips, I can see the shells potentially getting in the way of your thumb/hand. If you are planning on using one of the birds head PGs, it might have better ergonomics, but again,
I have no experience with either of them.
Vertical grip:
http://www.midwayusa.com/product/18...gton-870-winchester-1200-1300-synthetic-black
Birds head:
http://www.midwayusa.com/product/221333/speedfeed-pistol-grip-remington-870-12-gauge-synthetic-black
My brain drippings
One question, how are you planning on doing a reload from a side saddle, on a pistol grip only shotgun (PGO)? The pistol grip/butt-stock combos, have a tendency to use leverage against you, and make it harder maintain control of the gun during a reload. Without the stock, your going to lose a lot of leverage. If you come up with a viable reloading technique, I'd be very interested in it.
Hope this helps!
Chris "the Kayak-Man" Johnson