A good rule of thumb when shooting a recurve or longbow is appx. 10 grains per pound of pull plus or minus 2 grains. So if you are shooting 50 lbs. somewhere between 475-525 grains( including field point or broadhead) should do the trick. If your arrows are too light you may end up damaging your limb tips. Your arrow speed won't increase much with a lighter arrow anyway because of the physics of shooting a stick and string. When you shoot a lighter arrow, more of the string's energy is transferred to the limb tips and limbs. A heavier arrow absorbs more of the strings energy, and less goes to the limbs thereby reducing the chances of damaging your bow.
Also you will have to shoot feathers rather than plastic vanes.