How do I gradually change sprites?

Let's say I have 4 families of sprites and I want to spawn one sprite every x seconds. For the first 100 seconds from the start I want to spawn only sprites from the first family, then after another 100 seconds I want to spawn 80% of sprites from family A and 20% from family B, then after another 100 seconds I wanto spawn 50% of sprites from family A and 50% from family B and the percentage changes every x seconds.

How would you do it?

I usually do this with random(100) expression. For example, if I want to make 80% chance to spawn SpriteA and 20% chance to spawn SpriteB:

If random(100)<80 Spawn SpriteA
Else Spawn SpriteB

If random(100)<chanceVar Spawn SpriteA
Else Spawn SpriteB

start a timer at the start of the game then whenever you want to spawn an object, compare the timers time and spawn the object based on your comparisons.