Starcraft 2: Why use Roach over Hydra

First,

initial rush survival

Spinecrawlers and properly microed speedlings and roaches can really help in pushing back this initial rush. Force the AI to funnel up the ramp and place the spinecrawlers behind gas and/or an evo chamber. If you scout it early, set your speedlings outside your base so when they attack you get an easy flank. Use multiple queens with transfusion to keep everything going. Remember you can use them even on your spinecrawlers.

In regards to

What t3 units complement a hydra-heavy build?

I think there is a fair amount of discussion about this but I personally find a lot of use with broodlords. A tier 3 unit, which is very powerful at wiping a ground army. I prefer to use them in team games as well. Excellent base assualt as well as support.

EDIT: In a recent match between Idra and Drewbie(?), Idra tech switched from Ultra to Broodlord and ended up finishing the matched. Had he switched earlier, it is speculated that he would have much sooner. More power to the broodies!

Have your units meet up and wait for each other at various points along the way during your push, especially right before the attack so you can get in position. You just have to keep them together and tell them exactly where you want them to go. Xel'naga towers are good spots to rendezvous on your way over to the enemy as during that time period you get some extra vision which has saved me countless times when the enemy was also pushing as I was able to see them first and run my faster units back to my slower ones to intercept the enemy as a group. Even with all units of the same speed sending them straight into the enemy base in a line is going to cause trouble, you want to keep them all together.