I'm learning how to build out my squads and am noticing that there are many upgrade cards that are:

a) Used in builds but didn't come with the expansion pack I own. As long as I have the card I can use it? (obviously, as long as it is allowed with the ship card I'm using) For instance, I don't own Slave 1, but if my friend does, could borrow his "Gunner" upgrade for my Falcon build I can just use it? And if I don't have my friend around, I just can't use that upgrade in my build?

and secondly:

b) Does this mean upgrade and modification cards are available for ANY ship?. I ask because as I'm buying new expansion sets, I am keeping all of the cards that come with each ship together, and am paranoid about mixing them up with other cards. But more and more I'm seeing other ships with the same mod card. It seems that just "Title" cards are restricted to ships, and a few "crew".

Thanks,

Unless it has a header in italics stating something like "rebel/imperial/scum only" or "yt-1300 only", "small/large ship only" out are free to mix modifications, pilot talents etc. as much as you want.

For tournaments you need genuine ffg cards but the source is immaterial. You can borrow them trade them buy singles off ebay etc. For casual play most groups are fine with proxying.

Different upgrade cards come in each expansion pack and FFG encourages you to buy the ships that have the cards you need.

Unless there is a restriction based on ship type or faction, you can equip any upgrade to your ships that are allowed. Titles and Modifications are not shown on the Pilot cards, but you can have a title and Modification added to each ship. (For the Tie Interceptor, you can equip the Royal Guard Tie Title and it allows you to equip 2 Modifications on that ship)

A lot of cards are available to purchase on Ebay and at tournaments you can get alternate art versions of upgrade cards.

If you're playing casually with friends, you can Proxy cards you don't have, which allows you to try before you buy. I typically get an image off of the X wing Wikia website and print a copy for temporary use. Others create their list in a list builder program and print it to play casual games.

Official cards are required at tournaments, so keep that in mind when you go to one.