Marine xenophobia is dependent on the chapter, Ultramarines for example have fought alongside the Tau more then any other chapter. Other chapters won't fight alongside xenos no matter what. Thats also part of the fun of creating your own chapter/fluff.
As mentioned there will most likely be different "classes" of allies, and I honestly don't believe you will see any Marine players taking Necron allies in competitive tournaments because its just too risky. If anything IG or GK will be the #1 ally choice for marines, they have good units and are way more "trustworthy".
I believe ally slots are restricted to one of each FOC, so I think this will limit abuse. Your not gonna see a Marine army with 9 crisis suits thats for damn sure lol.
Tyranids are the only race that can't take allies unless its other tyranids. It looks like if you ally with your own faction then you receive an extra slot for each FOC. "Excuse me sir, can I order 3 Hive Tyrants and 4 Carnifexes with extra butter and mayo?" Yes please haha
This is what it looks like based on rumors, past rules, GW website, and the latest White Dwarf. We will know everything for sure in 6 days. Can't wait!