Going to the point. 
 
First question. There are some abilities like decryption that require a minimum level to be used at some points (to open locks, etc.) I was wondering if that level is the maximum of the squad or the sum of that ability in the squad if several people share it. To put an example, let's say I have Tali with decryption lvl 5 and Garrus with decryption lvl 3, the decryption level of my squad is 5 or 8? 
 
Second question. Some abilities are passive, specially in engineers, and for instance increase the strength of the shields. The question is: which shields? If I'm a soldier going with Tali and Garrus, are my shields increased by the abilities of Tali and Garrus or each one only affects their own shields and mine are unaffected? 
 
Thank you.

 I'm pretty sure you use the skill lvl of the squadmate with the highest level. So, in your example, your decryption level would 5. 

Yes, I was guessing that, about the abilities that may change some properties (like shields) of a member of the squad or them all, I'm guessing it's only one, but I would like to know, since that changes the strategy completely.
 
Thank you.