There is a wonderful sound to this set of rumors. I am speaking of the terms used in this set, "Every Unit Viable. Every Unit Desirable". Phrases like this being used as a design parameter (figurative of course) are always some of the best things to hear from close sources. Grab some morning coffee, and sit down and enjoy this set, as its a great start to the day.
Please remember that these are rumors, and are more specifically playtesting bits.
via a strong anonymous source
Some more specifics on tyranid playtest.
Carnifexes are in the elite and heavy support sections (but have different options)
While broods are the same base cost as the current dex, additional gaunts are dirt cheap. So if you want to horde it up, you can.
Rippers are cheaper.
Ability to build around "free" units coming in from reserve throughout the game (from different sources, but it's the same special rule), but they do not score.
Can go the other route and bugzilla it up and have all (including scoring) monstrous creatures.
The goal is to go mass or massive or anything in between.
The big text on the design board (figuratively speaking) is "Every Unit Viable. Every unit desirable."
And on the topic of evolutions - these are just upgrades and options that can make a bug very finite in it's role, but awesome at it. Not mid game changes. Only one special creature can do that.