March 2013 Model Rail has an interesting article call Trains to put on your shortlist.
Which gives a lot of steam era short trains including pictures of the real ones and which manufactures of loco and coaches that would make up the trains.
Including some combinations we probably would not have thought of such as a BR 14XX and a Thompson non gangwayed brake second.
So my thoughts are steam era.
As there are some interesting and easily put together short trains that could used in a rural setting.
and its possible you may already have some of what's needed for some of the illustrated trains.
The article states that most of the trains were replaced with DMUs.
However I have seen a picture of an 04 or was it 03 shunter with a MK1 composite and MK1 brake second
which ran in a rural setting as a regular train service.
Which I have based one of my trains on using Bachmann crimson mk1's and green 03 or is it 04.
Given your small layout and its limited stock capacity it is very tempting to say the Answer is D All of the above
Because it would not take much to cover all options allowing a variety of eras to be covered.
Making it possible to change the layout completely just by changing which trains you run that day.