For the bottom beads, yes. thumb for adding, forefinger for subtracting.
But for the top beads (number 5 beads) we use only the forefinger for adding (push up) and subtracting (push down).
At the early stage (especially with younger kids) The goal is the ability to translate the abacus beads patterns into a number (and vice versa). So I think showing big numbers should not be a problem. It's just like how we flash words when we teach reading.
We can always worry about the formulas later on :P
Start with showing single digits, and then follow with 2 digits, then 3 digits and so on. if your child knows +1s or -1s.. then show the cards in
order. 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,..etc.. 100,101,102, etc . Show as many as you can, as much as your kid wants!
Make sure it's fun
If you wanna check for understanding.. play number games with the abacus cards!