You’re clearly concerned about shrinkage and quite rightly so.
It all depends on how dirty and what the primary colours are and that and not the carpet type that are the deciding factors in choosing your pre-spray, because your job is first to get the carpet clean and second to do it without it shrinking. You want a pre-spray that will quickly and effectively loosen the soil, preferably with little or no agitation other than with a carpet brush and virtually no dwell time as that always the moisture to penetrate the backing. Powerburst is the best for this as it works practically on impact.
The backing fibres (the ones that shrink) tend to have relaxed a lot with age, you can tell that if you kick forwards on the carpet and see a bit of movement at the front of your foot. Newer carpets tend to be still very tense as that is how they are fitted.
Old carpets are worn and so the pile density and the wools ability to absorb moisture before it reaches the backing are considerably less than with a new carpet.
If the carpet is older and has some flexibility in it then when it is cleaned any moisture that reaches the backing will be taken up by the looser backing fibres before the carpet actually shrinks.
On a newer carpet the carpet is already tense and if moisture reaches the backing there is a greater risk of shrinkage because there is no slack to be taken up.
So in either case it all comes down to how well the carpet is fixed on all four sides and the strength of any seams, which you should check thoroughly regardless. You should not be able to lift the carpet off the gripperods, or lift any seam.
There are risks with any woven carpet, regardless of age, the trick is knowing how to manage them.
Well, you did ask?
Simon