Mass-effect – What are all the models that Shepard finds for

To be awarded this medal, the player's team (yes, the whole team counts, as it is a squad-medal) needs to kill 5 enemies or more in 1 second. So it can be done with a rocket launcher of course or even when everyone of the team kills an enemy and one player has to kill two with one shot (which can easily be done with weapons like the Widow or Javelin) and the shots have to be fired nearly simultaneously!

You get 2500 points for the bronze medal (1 killstreak), 5000 points for the silver one (2 killstreaks) and 10000 points for the gold medal which is equal to 3 killstreaks!

There are definite synergies between various classes and certain weapon types. First of all, many powers and abilities only work with specific weapons, or give special bonuses when used with specific weapons. For instance, Vanguards have access to a number of bonuses to Shotgun damage, and Soldiers have access to several Assault Rifle upgrades. Infiltrators have access to lots of Sniper Rifle synergies.

Additionally, only Soldiers and Infiltrators experience the 'time dilation' effect when looking down the scope of a Sniper Rifle - this is invaluable for setting up headshots.

Finally, different classes have different weight allowances. Because some weapon types - notably Assault Rifles and Sniper Rifles - are much heavier than others, trying to carry one with a class with a low weight capacity (like an Adept) will result in a large penalty to your power recharge times. This is an important tradeoff, since these classes essentially live and die by effective use of their powers.

TL;DR: While you aren't required to use certain weapon types, there are definite rewards for specializing in something suited to the combat style of your class.