I'll try to answer some of your questions.
First the Thunder 9 UC is the compact version of the Bersa Thunder 9mm HiCapacity.
The older Thunder 9 had no accessory rail on the dust cover. This is true for the UC and Hi Capacity models.
This is the older 9UC
This is the older Thunder 9mm Hi Capacity
The Thunder 9mm HC Pros are as follows
Thunder 9 UC Pro
Thunder 9 Pro
Standard Thunders had standard barrels and came with one magazine.
All Pro models come with match grade barrels and two magazines.
If you are looking for a slim 9mm for CCW consider the soon to come Bersa 9 BPCC
as seen below.
I believe it is an 8+1 single stack mag.
I know I will be looking at this when it comes out later this year.
Hope this helps.