To use the best weapon currently available in each category, you need about 60-70 points in the corresponding skill. If you don’t have enough, the game won’t let you equip the weapon at all. Additionally, there seems to be very little difference in performance between having 70 in a weapon skill and having 100. If you restrict your skill in each weapon to the minimum needed to wield the weapon you like best in that category (which may not be the top one) you can potentially distribute your 150 points (3 points per level over 50 levels) in such a way that your merc can use 3 weapons reasonably well.
Which weapons those should be is entirely up to you.