overwatch works well at times, but could be more usefull. u activate it by right clicking on the unit upgrad instead of left clicking. it'll then flash on off to let u know its working. When the upgrade is full it stops. It dosnt work if it can only build 1 thing and so is excelent for creating squads, hitting 'overwatch' on troops and guns and then leaving them to run to ur frount lines.
never had any problems with the zoom really, make sure u have 'full 3d-rendering' on to really be able to move the camera to worthwhile views.
as to the balence of troops the nearest i think it comes to is the eldar, who are just specialised to deal with the threats. but it still comes down to building as much as poss and if u build the wrong things then just delete em to get the right thing