How are you attaching the fridge cabinet to the wall? It needs to be attached firmly to the pantry and a panel on the sides in order to bear just it's own weight without anything else. Standard Blum drawer hardware usually will hold 75 lbs, including the weight of the drawer/pullout. If you glue and bolt the cabinet together like a factory cabinet would be assembled, it will contribute to it's rigidity and weight holding capacity. No numbers to that though.

An over the refrigerator cabinet is supposed to be attached on at least 3 different sides and multiple points per side. So, both sides and the top, a side, the top and the back or the back and both sides.

So, you likely need a panel on one side and then the over the ref cabinet and then the pantry. The panel is attached to the base cabinet side and the wall cabinet. The over the ref cabinet is attached to the panel, the back wall and the side of the pantry.

There are a couple of other issues to think about - the location of the electricity (so the unit can be plugged in but not have the plug push the ref too far forward off the receptacle), the depth of the ref and how built-in you'd like it to look (in case you need to pad stuff forward a bit), plus keeping the ref doors in front of the panels and the pantry doors. Also consider if the pantry doors bind on the ref doors and check that the handles on the pantry can't be hit by the ref doors when open.

Also the height of the ref door hinges and whether your total ceiling height allows for the cabinet size you've chosen plus a little extra for those hinges, leveling the ref and leveling the cabinet run. The ref manufacturer will specify some gaps for air circulation, check those too.

If you think you'll bend the cabinets or panels with the weight load, attach some 1/2" ply to the back of the pantry cabinet in front of the dust cover that ikea supplies - meaning inside the cabinet. It needn't be complete - more like a 6-8" tall strip at the top and bottom and then in middle. Ikea drawers are not full depth so their function will not be impacted in any way. Just screw right through it when installing the cabinet.

Another weirdo thing to remember about tall cabinets like the pantry. You need more height than the height of the cabinet to stand it up. Double check that before attaching the feet.

If you've allowed for structural stability as GreenDesigns and bmorepanic explain, then there are no worries for adding extra drawers. I have six drawers in the lower section of my pantry. I sized them to fit my canned goods and other storage items.

The over fridge cabinet and top of the pantries are the ideal places to put pullout Ikea wire pullouts. I have one in my basement kitchenettel You can see at a glance from below what the contents are, and poke/push contents through the bottomwith your fingers until they reach the edges where you can grasp them and pull them out. You can't see the contents or push things out of the middle with regular drawers because of the solid bottom.