This is an old one, but I figure it's worth a ressurection....
Go to non-integer sizes for beam weapons, and build the power plants into the design. In other words, in the design window you would specify the recharge time (bounded below by the race's best recharge rate) and Focal Size (and reduced size if any), then Aurora would build a big enough power plant into the weapon to recharge it over that amount of time.
In the example in my previous post, for the 15 cm laser (power req of 6) and C3 recharge technology, you'd be able to choose recharge times of 10s, 15s, 20s, etc. (10s = 5s*(6power/C3)). If you chose 10s, it would build a power-3 reactor into the mount; assuming Gas-Cooled Fast Reactor Tech (4.5 power/HS), the size of the mount would be 4.66 (4 for the laser, plus 2/3 rounded down for the reactor). If you chose 15s, it the size would be 4.44 (Power 2/Reactor 4.5=.44 for the reactor in this case). Note that I'm rounding down so that 3*(2/3) fits into 2HS; you seem to have already done this for reduced size launchers....
For the 20cm laser (power req of 10), it takes at least 20s to recharge at C3, which corresponds to a recharge rate of 2.5/5s. So the size would be 6.55 (6 for the laser, 2.5/4.5=.55 for the reactor) with a recharge time of 20s.
John