Having a look at the seating for the ride while I am in the queues allows me to assess the leg room and ask to be seated wear I think the most leg room is. Sitting at the front row doesn’t always give you more leg room as sometime the second row has a space under the seat in front of you that allows for more leg room.
WDW used to have a FAQs thread on the official website. One of the questions had to do with which rides had more leg room for someone with a leg cast. They took that off their website a few years ago and I have not been able to find information about it.
[/quote]from Universal website
Any guest with prosthetic arms or hands will be required to satisfactorily secure the limb and grasp the ride restraint. If you have any questions, please ask a ride attendant or visit Guest Services.
Guests must remove prosthetic limbs before riding Dueling Dragons or Pteranodon Flyers to prevent hazards or loss due to ride forces.
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Do you think these rides will have a seating area close to the ride boarding point? ) [/quote]
If you have not seen it, there is a downloadable "Rider's Guide" for people with disabilities on the Universal website. It is actually much more informative than Disney's guide for their parks.
For your specific questions, you could ask on the Universal Board or try emailing Universal if you don't get an answer here.
Someone posted this contact information for Universal on another thread:
guestservices@universalorlando.com Or call guest services at: 407 224-4233 Press option 9 to speak to a Guest Service Coordinator.
Am correct in assuming that not all rides have a disabled entrance and do people with GAC but not in wheelchairs use these entrances wear available?
As was already posted, you are correct.
For the Disney parks, AK and the Studio were built with Mainstream Lines, so at those parks, people with wheelchairs are pretty much in the same lines with everyone else.
MK and Epcot are older parks and have some attractions that were not able to have Mainstream lines. Some of these are because the line itself is not accessible. In some cases, it's because the attraction has the entrance and exit at different places and people using wheelchairs or other assistive devices will need to board at the exit so that their assistive device is waiting for them when they get off.
Universal was built after Mainstream Lines were common and their attractions pretty much have Mainstream Access.
And is it expectable to use the fast pass line to enter a ride using my GAC when there are no fast passes available for the time we are in the park?
All the Fastpass lines are accessible, but some of the regular lines are not at Epcot and MK. In those cases where the regular line is not accessible to you, you would be able to use a GAC (Guest Assistance Card) to access the Fastpass line if there are no Fastpasses left. You may be taken right in, or given a card with a handwritten return time (usually similar to the wait in the standby line).
That should also be the case with the situation you mentioned for Big Thunder Mountain RR. If a handwritten fastpass is not offered in cases like that (and they just tell you to go in the standby line), you could try asking for a return time. (Also, by the way, there is a wheelchair boarding area for BTMRR. We have never used it because it would not be safe for DD to ride that, but I people have posted about it).
I know people have used cane/stools
like this one. It would not work as a plce to sit to take off your leg and I'm not sure if it would work out for you as a place to sit in line, but it might be worth a look.
and as my daughters are starting to get older they want me to go on some of the bigger rides 
with them. But I noticed on our last vacation that most of the bigger rides or rides wear your legs just hang have restrictions not allowing me on.
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