Love the railgun and the decoy.
For the railgun, I think it should be the same size as the ion beam, not 3x5. Make it basically the physical equivalent to the ion beam. The difference is the high alpha for railgun. Ion beam should be higher sustained dps, whereas railgun should do less dps, but far higher burst damage. I like to imagine a tandem railgun salvo of 3 shots - it slams through one layer of shields with two rounds, and the third penetrates deep into the hull in a line. Also, make the railgun use the same ammo as the other guns - just more of it per shot, maybe 8 ammo, just like the ion beam takes 8 batteries. Lastly, in keeping with the design of the other physical weapons, it probably shouldn't use energy at all.
The decoy or something like it is absolutely necessary. I have a missile battleship that fires 72 missiles at once. To be able to defend against that many missiles, you would have to sacrifice half of your ship surface to point defenses.