Hello,
TL;DR available.

I did the Alien Pack rebalance for my own playthroughs (imo new aliens are mostly great but poorly implemented) and want to share it with you. Basically, it is quite large overhaul aimed at legendary difficulty, however it is more of "vanilla with interesting twist" than "totally new game".

My goals were to make early game comparable to vanilla in difficulty, maybe a little bit harder, and add more difficulty to mid and late game. At the same time I wanted to preserve important "alien moments", i.e. keep vanilla weights high when new important aliens first appear (e.g.muton, archon etc.), and not decrease Advent Captain appearance to much in mid-game when skulljacking him is a consideration.

Total Alien Pack aliens weight in Follower Pool is 20-40% depending on forcelevel and in Leader Pool 10-30% depending on forcelevel (in basic Alien Pack it is even 40-60%).

Gunners are eliminated, since I consider reliable one-shooting soldiers in cover too early in the game a bad design and Gunners do not offer much in terms of interesting new mechanic. Naja appearance is a little bit delayed and large bunches of them will be less common. Stronger versions of basic Viper are eliminated to not increase Alien Pack weight too much and they are not really necessary when other, more interesting strong aliens are present (who would want slightly stronger Viper when one should be fighting muton elites, sectoid commanders, sectopods or andromedons?:) ).

Here is the link to pastebin:
http://pastebin.com/93QTXF23 (updated 20160831)
Just replace current contents of XComGameData_CharacterStats.ini with this. Enjoy:)

It is work in progress (although playtested) and I appreciate your feedback. New versions may appear later.

Have fun:)

TL;DR: Alien Pack rebalance available (less Naja frustration but still hard, especially in mid and late game) - Link: [url]http://pastebin.com/93QTXF23/url] (updated 20160831)