What would help most is if you posted a replay or two for people to check out. Here's some general advice (I'm at about the same level)
1. Just a handful when you see them should be plenty - the trick is to focus fire the colossi down, and don't fly right into a big pile of stalkers to do so. Get a good angle. Obviously, he knows your vikings are going to cause him a problem, so that's what he's going to target. Also, split your units appropriately for storms, etc etc
2. 1/1/1 w/ banshees, 3 rax are a couple
3. Harass consistently (drops, banshees, reapers, hellion run bys), so that he 1) is afraid to move out 2) has to split his army up around his bases to cover. Protoss have this death ball style of play where for the most part their army wants to be grouped up. Also, at gold league in general, people aren't proficient at multi tasking (and it's still a challenge all the way up) - multi-pronged attacks are definitely something to work on. When a protoss turtles, that means you are free to move around the map - do a little damage here and there and don't be afraid to run away. Take a bunch more expansions, take the gold base/s, and build your economy and structures such that if you lose a battle, you can remax your army in no time.
4. Hellions are great for taking out workers, and not much else. Theoretically zealots and sentries although they might have 2 much health - I don't know.
What unit composition are you going for? Marauders are fantastic against all tier 1 units and buildings, and protoss generally eat any kind of mech play alive. A handful of stimmed marauders will take down a cannon like it's nothing.
My advice is to make sure you can do damage to his economy - banshees, drops, hellion run bys, anything to kill off probes. Killing workers is the simplest way to get an advantage.
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