You can set placement radii on waypoints as well.
A good trick is to create an AI group. Name the leader, say loon1. Give him a probability of presence and obviously a placement radius. Create a large group, with a good variety of weapons. Each loon should have a probability of presence. Also, if you look carefully, you will see that each unit has a Condition field. In every Condition field except the group leader write this
alive loon1
The effect is this: the group leader may or may not be in the mission, depending upon the probability of presence. If he is, he could be anywhere in the placement radius and his group will be in formation round him. If he is not, none of his group will appear since their probability fields apply only if the condition field is satisfied. And, thanks to those probability fields, some of the group members will appear and some will not.
So you have a group of random composition which may or may not appear anywhere in the placement radius.
To do the same thing effectively with tanks/armoured vehicles you need to add a step. You can put only 4 tanks in a group before the game gets stroppy, which is no good if you want a 4 vehicle group of random composition. (Say a mixture of Abrams and Bradleys with the possibility of a Vulcan. The four vehicle thing is based on each one having a crew of 3.) The solution is to create several groups, using the same tricks as above. Make the first waypoint join and synchro all the join waypoints together. The game is clever enough to work out the maths so that it creates a maximum of four vehicles. If you've got APCs in the mix then of course you can create infantry groups as above and assign them to an APC. Use condition of presence to ensure that you don't have too many soldiers for the wagons.
To help your long range AI snipers (who frankly are probably good enough already if you've got their positioning right - you have to Preview the mission as each one to check the field of view is correct) use the command reveal. For example, create a trigger around the sniper, size as you like
Activation: anybody present Repeatedly
On Activation: "_x reveal sniper1" forEach thislist
syntax not guaranteed. This will mean that the sniper knows about everybody in his area, so he will shoot the enemy if he has a shot.