as for question 2, tankers are not worth it as fuel has no use in the game currently, so it's always best to sell 100% of your fuel and then you'll have plenty of room in your fleet for any you capture.
freighters are hit or miss I'd say. It depends a lot on your fleet composition, what targets you're going after, and your play style.
For example, if you plan on fielding a lot of fighters, it can be useful to fly a freighter to store extra supplies for you. This may not be necessary if you use ventures (or an atlas!) as your flight deck as they have a lot of cargo space already.
If you plan to raid station resupply fleets which generally drops a lot of weapons and supplies, again, useful to have a freighter.
Finally, if you plan to capture a lot of vessels or in general your play style ends up with hull damage more often than not, a freighter to hold extra supplies will get you through a few battles without having to resupply.
I can't think of many other cases where a freighter is useful. In my game I did buy an atlas just to shuttle a few thousand supplies to the abandoned station so I could fix up any captures I had before mothballing them. I subsequently mothballed the atlas
