First, clarifying some terms: MOLLE is the modular gear system. PALS (Pouch Attachment Ladder System) is the webbing "grid" that is used to interconnect the modular MOLLE gear.

1. Can I mount Lowepro SlipLock cases / pouches to PALS straps on MOLLE gear?

   No. Not without some sort of adapter.

   On MOLLE gear, the PALS webbing is 1" wide and spaced 1" apart, and attached/sewn to the bag or pouch every 1.5" along the strap. On the Lowepro case, the reinforced tab that slides through strap webbing is (I think) 2.5" wide, meaning it's too wide to slide into any PALS loops.

   A "MOLLE Ka-Bar" adapter, which basically provides a belt-sized loop to mount older Ka-Bar sheaths to MOLLE gear's PALS loops, can provide a large enough loop to mount a Lowepro Lens Case to MOLLE gear. They are plentiful and cheap on eBay, various online sellers, or at a Army/Navy surplus store. The lens case will probably sag a bit or flop around, though, because of the extra cloth interface and looseness of the adapter's loop.

2. Can PALS addons be attached to a Lowepro SlipLock loop?

   Yes, with caveats.

   If you look at Lowepro's SlipLock loops on any bag or case that has them, you'll see the webbing sewn at alternating intervals of 1.5" and (I think) 2.5". The 2.5" loops obviously receive the SlipLock accessory cases and pouches. Compare the webbing pattern on the ProTactic BP 250 AW (first image) to the PALS webbing spacing (second image):

   
   LowePro BP 250 AW, from Lowepro.com

   
   PALS webbing spacing, by Einfassband from Wikimedia Commons. CC-BY-SA 3.0

   MOLLE gear that only uses a singe PALS loop, or 2 adjacent PALS loops, would connect just fine to the Lowepro SlipLock loops.

   MOLLE gear that uses non-adjacent PALS loops (i.e., skipping one or two PALS loops) would need to be attached through the wider-spaced loops on the SlipLock gear, possibly sagging more than it would if attached to PALS-spaced loops.