The platforms at Thalwil have been somewhat controversially shortened to 300m usable length during level-boarding platform height adjustments (there is a very active rolling programme of rebuilding platforms to the Swiss 55cm standard, due to equality legislation that comes into force in January 2024 that requires unassisted boarding for passengers in wheelchairs to be available at all stations).
300m is the standard length for regional services, 400m for intercity and international services (this refers to the maximum permitted train length, the platforms are of course slightly longer).
The SBB have been gradually moving away from strengthening services in this way (using "modules" and individual coaches), with EMUs at standard lengths being coupled instead.
The new FVD trains are mostly 200m long, but there are also 100m units precisely in order to be able to lengthen sets where necessary. This is also slightly controversial due to the two extra cabs required that don't do anything as well as the reduced seating capacity in the FVDs compared to IC2000 per coach. These 100m trains aren't supposed to ever be used on their own, but this has been known to happen during disruption and during the times where the FVD reliability and MDBF (mean distance between failures) was extremely poor resulting in poor availability.
The IR service between Zurich and Luzern has extremely high demand and this new, artificial length limitation to 300m is beginning to prove somewhat problematic.
The IR36 does have a lot of variation in the rolling stock used - I've seen everything from EW IV + Re 420. ICN, FVD, and RVD on the route. The new Giruno sets have also been used for shakedown runs and driver familiarisation upon entry to passenger service after delivery, before they were transferred to the international services to Italy.