All multiple RTRs and F-RTR can be stacked multiples of 3, 6, 9 etc.etc. For example communications(multiple of 9) x18 or x36 or x99. Sure, F-RTR well we are supposed to do 3-5 F-RTR niners or 27-45 reps stacked. Tetra is kept at the same level not increased.
For 88/99 W/B awakening rituals no the 88 and 99 are specific to produce certain effects of that numerology.
Other than 88/99/Tetra you can stack any of them. For example the shortest RTR is the single sentence Anti-War RTR which can be done 108-216 in like 4-8 minutes or so nearing 10 minutes.
As for affirmation keep it the same so stack the words but keep the affirmation 3, 6, 9, or 18 depending on the RTRs.