I wouldn't say that ASFs can't overheat, because they can... it's just a really, really bad idea.
The heat scale is far more unforgiving for ASFs than it is for BattleMechs. If you overheat in a BattleMech (around +9 or so), the worst you'll suffer would be loosing some speed and a targeting penalty. The ammo explosion check doesn't come until somewhere in the teens, I believe.
For an ASF? Overheating that badly forces you to make a control roll. If you fail, you go Out of Control, which means you're locked onto whatever vector you were going, and can't fire any weapons. In space, it leaves you a sitting duck that's good for nothing but enemy target practice until you cool off. In an atmosphere, I'm pretty sure it also has a pretty severe chance of meeting the ground in a terminal fashion.
Now, as for the ERLLs, it clearly requires strict discipline. A 4-3-4-3 pattern would be required to keep the heat under control. Now, as for modifying it, I question the SPL in the nose. And why an ER Medium paired with a SPL in the rear? Personally, I would delete the ER Medium, move the nose SPL into its spot, and add another heat sink with the free ton.
That way, the overheat is just +4, which isn't as bad, but improves your safety margin when watching your heat gauge. I would heavily recommend dropping one of the ERLLs when using the rear guns. Assuming you were on the 4 portion of the pattern the previous turn, you come to exactly 0 heat when firing three ERLLs and both SPLs.