Because railguns fire 4 projectiles, they are equivalent to lasers for PD (which have a 4x bonus to-hit from turrets). However, since railguns do not require the additional turret mass or high-speed fire controls, they end up being better than lasers as PD on a mass for mass basis. That said, lasers are much better offensive weapons, so they are superior if you only want to focus on a single type of weapon for both offense and defense.
The speed of your ships is pretty good for nuclear pulse engines. I typically consider the "standard" speed for an engine the speed of ship with 25% of its mass devoted to an unboosted engine (corresponding to the FC tracking speed techs), which for nuclear pulse engines is 2000 km/s. Beam ships need to be faster to catch their enemy, so I would say anywhere from 2000 km/s to 3000 km/s, which you've already achieved. Like Cavgunner stated, fighters are difficult to do well at this tech level, since you don't have the engine boost techs to make them more than about twice your mainline ships' speed, which limits their usefulness as a combat vessel. I do like the idea of having a recon fighter, though, both for roleplaying purposes as well as the general utility it could provide. I would say drop the laser and up its fuel and deployment time, since it seems unlikely you will be able to detect or engage any enemy fighters before they can track your capital ships. I really like this fleet, though, and I would be interested in seeing some of their opponent's ships.
One last suggestion: if your factions don't have jump theory researched yet, you could probably cut down on the fuel reserves, maybe to about 10-15b km, which is plenty of range for any battle within the orbit of Pluto.