There's
no definitive mix of units that will lead to a victory in
Z:SS. As for robots, up to patch 4, there's just not too
much reason for psychos or pyros building as a mainstay
unit, just build toughs for your mobile robot army. Nevertheless,
you should use Pyros for bridge guarding (when you want
it to stay destroyed), and to take out heavy anti-tank gun
defenses. Psychos are great as a partner for the deadly
mortar move-shoot tactic. But for christ sake, don't build
psychos thinking they're marines and mortars thinking they're
siege tanks, or you'll suffere a miserable death. This is
NOT Starcraft, ok??
Lasers are weak for their cost, so they should only be used
on heavily unaccessible maps where they are easy to be kept
out of enemy LOS up to the very last moment. As for the
tanks, it really depends on the map since the tanks are
heavily hampered by the terrain and the amount of resources
the map offers. Read the Z:SS unit tips for more info.

A
super effective attack group in Z:SS.
(Click
to enlarge)
The
above is a good real life example: group1 = toughs and a
couple snipers, group2 = medium and heavy tanks, group3
= mortars, group4 = jeeps and light tanks, group5 = spies.
The jeeps and light tanks can take out enemy mortars and
protect your own when used correctly, and light tanks are
specially effective against toughs. Try moving your spies
really ahead of the attack group to save him from friendly
fire. If enemy has spies, you'll lose them and have to rely
on your snipers. When possible, get one spy guarding the
mortars to give them the sight to be able to defend themselves.
If you just want the snipers for their sight range and don't
want them running into enemy fire, set them to neutral before
the combat. You can still target them manually.
Next:
General Techniques