Others have already hit on some of this, but as one of ENWorld's big monster-building types, here's my critique.
Silt leech:
Should have a role- “minion skirmishder” is a good choice.
Non-AC defenses are a little high.
Minion damage should always be static- for a level 2, it should come in at about 5 damage.
You absolutely should not have a recharge power on a minion- it is simply too much to track in a typical battle involving minions. I'd change the static leap power to more like this:
Static Leap (standard; at will)
Attack: Ranged 10 (one creature); +5 vs. Reflex.
Hit: 5 damage, and the leech shifts to a square adjacent to the target.
Miss: The leech shifts to a square 3 squares past the target.
Hydrostatic Mitosis is an awful power. For one thing, you have to track the damage dealt by each of the leeches- and as minions, there could be a TON of them. I would drop it entirely, but if you must have some version of it, drop the tracking damage thing. Also, you should probably specify whether the newly created leech counts for xp in the encounter (I'd say yes).
Silt slug:
HP should come out to 38 (3x8 for role + 14 for con score).
Static boost- This is problematic imho, especially because of the stacking clause.
No basic attack, huh? I think he needs one, or else his damage output will be crappy.
On the other hand, static flash's damage is too high IMHO. 2d6+2 looks about right.
Statis push- your “Special” note makes me think you missed the rule about entering hazardous terrain. You get a save to fall prone instead. I'd drop the special line you have entirely.
Static Arc- this power is way more confusing and complex than it needs to be. All this “straight line” this and “no forks” that stuff should go. It needs to be useful, not fancy. The automatic hit seems weird- why? Also, I strongly recommend against setting damage based on the number of targets hit- 4e pretty much demands that you stick to the numbers by level for the game to come out well-balanced. Finally, sliding in a random direction just seems weird- let the silt slug control the slide.
Brooder:
My quick in-my-head calculation says she should have 100 hps (6x6 for role + 14 for Con)x2.
Leech Brood- do these count as part of the encounter's xp?
Static Sling- Even as artillery, an attack vs. Reflex should be at about lvl + 5 at highest. I would leave out the “and effected by the silt leech's dehydrating body” line since the leech acts on its own next.
Static Stun- Stun (save ends) powers at heroic level should pretty nearly ALWAYS be encounter powers IMHO.
Static Vacuum- should SLIDE leeches into the brooder, not shift them.
Static Overload- so a fully loaded brooder does 14d10+14 damage when it dies? Jesus. Leave off the extra damage for the leeches or change it to like +1 per leech or something. That average of 91 damage may well TPK most lvl 5 parties.
