what's the lowest possible increment in uS units of the two other timers with 20mhz quartz?
i think thats the wrong question

1. first thing is what time intervals are required to toggle the output at to achieve for the given number of pulses in the time frame
2. are they all possible within the available timer resolution ? how many pulses do you really need ? steps of 5 10, 50 ,1 ,2 ?
3. what method will be fast enough to toggle the output and count the pulses too .ie an isr or a fast loop to monitor timer overflows

PULSES TOGGLES PERIOD uS
100 199 1204.52
101 201 1192.54
150 299 801.67
200 399 600.75
250 499 480.36
300 599 400.17
350 699 342.92
400 799 300.00
450 899 266.63
500 999 239.94
550 1099 218.11
599 1197 200.25
600 1199 199.92