My advice would be to put Al'rahem on foot because it's easier to lose five guys in a group of 55+ than it is to miss a chimera, take the full five squads and put a heavy weapon in each one. Maybe throw in a few special weapon squads with melta or plasma if you really want to. That way you have a group that can come in shooting, and can be a group that your opponent has to either confront with a lot of units(leaving your main force unopposed), confront with a few units(Al'rahem's platoon takes them down and links up with your main force), or ignore them(your main force takes a beating, but you get to really have fun with Al'rahem). I'd stay away from heavy weapon squads. Unless this is your only platoon, you'll want them firing every turn, even though you can more or less guarantee that the platoon will be coming in on the second turn by using an astropath.
Yup, that sounds about right. The only faction who are a bigger bunch of backstabbers than the Imperium are the Eldar (and this is debatable as the Eldar only stab OTHER factions in the back).