You can balance ammo with some quick math and some rules of thumb.

Every 'BIG' box of shells counts as 1000 points of damage. 56 dmg per SSG blast times 40 shells divided by 2 shells per shot, rounded down from 1120 just to fudge a bit for missed pellets. assume an equal rate for the SG, since even though it's actually a little weaker than half a SSG it's more accurate. We're going with an assumed maximum damage of 25 HP per shell.

Every 'BIG' box of nails counts as 450 points of damage. (9 per nail/18 per supernail times 50 nails.)

Rockets are harder to gauge, because splash can potentially do a lot of damage to a lot of clustered targets, but call one rocket ~180 damage, on average. A 'BIG' box carries 10, so that's 1800.

LG, 30 damage per cell, 12 in a 'BIG' box, that's 360 more.

Add up the damage that can be done by all the ammo pickups in your map. Add 360 for every ogre, since they drop rockets in twos, 125 for every grunt and 150 per enforcer (you didn't have any), then add another 625 for the player's starting 25 shells.

Now, add up all the monsters' health. You see where I'm going with this.

You now have two numbers. Are those two numbers close to each other? Is the correct one larger? Is the ratio between them appropriate for the difficulty of your map? Is that ratio steeper on Easy, or on Hard? How much infighting is possible in your map? How much infighting do you want the player to actually rely on?