I can't seem to find any advice for troop combos, so I'll write one here. (Note: I am an HQ 8 and thus don't know about any combos above that level)
Rangers are your basic infantry units, and essentially form an all purpose army in and of themselves, with rocket launchers, medics, and gunners. Ranger take up one tarmac slot. The are very effective against defenses that focus on killing single units, but falter at the sight of artillery, commandos, tanks, tomahawks, and land mines. They pair well with tomahawks and tanks, which increase the army's damage output, and strikers, which sheild rangers from danger.
Strikers have a lot of health for their size (3 slots), but also have a low damage output, so you may want to pair them with another unit, like rangers, tomahawks, or tanks. All-Strikers is also used occasonally, and can work in absence of spider drones.
Tank-Based Armies: The tank is a powerful armoured vehicle that is very strong against guard towers, sentriy domes, and infantry. However, they take up twice as much tarmac slots as a striker, so you won't have many. They pair well with rangers and strikers. My personal favourite is Tank-Tomahawk.
Attack drones are powerful air units that swarm through a base, targting its defensive structures. However, they are very vulnerable to non-THEL domes and patriots, although other structures can't attack them. Infantry are also a problem. Attack Drones don't really pair well with other units, mainly because they move fast and are vulnerable to different things. On the other hand, you could use drones to clear out a large area of non-air targeting defenses.
Tomahawks do immense amounts of AoE damage, but are very easy to kill. Hide them behind other unit like tanks, strikers, or rangers.