The UTH can be mounted on the right, left or center.... It needs to be in the center if you are using a cage divider so both sides of the cage get heat. The standard way AP sets up cages is to put the UTH on the right side of the cage. (I had to ask them to get mine with the center mount)
I have both the T8 and T12. I would stick with the T8 for BP's but there is nothing wrong with the T12. Be warned though, a T8 will be shipped to you in about 10 days - 2 weeks but a T12 will take 6-8 weeks.
regardless of cage size the same principals apply. Things like Radiant heat panel wattages may vary (taller cage = more air to heat)
also if you get the radiant heat panel ahead of time, and have it shipped to AP they will install it for you and make a nice power cable hole.
I like having the lights in them, but they are not 100% necessary.