If you use an A2 barrel on an M4 upper, the lips of the barrel extension protrude over the ramps machined into the upper where the barrel/upper mate - that is not good. The bullet tip can catch and cause a misfeed. It's easy to match an M4 upper to an M4 barrel (current Mil std) at no extra cost...doing so will eliminate any problems. Take a look at this:
http://www.m4carbine.net/showthread.php?t=193
The problem with using a rail mounted front sight on a carbine length fore stock is the sight plane is significantly shortened between the front/rear sights. This directly affects accuracy. I highly recommend using a gas block front sight on a carbine length weapon...either std A2 or a gas block folder you can stow out of the way of the optics on a flat top. You can use the MagPul rear sight, but there are far better ones available. Think of the sighting system as the heart of the weapon - you do not want to go cheap here.
It's ok - I've been building AR-15's for 25 years