While the splines should be dry I like to make sure the clutch plate can easily slide on the splines and I put a bit of Never Seize very sparingly on the splines and wipe off any excess. It can get hot and you don't want any contamination on the clutch plate so never use grease. I also use a spray of Brakeclean on the flywheel and pressure plate just before I put it together to remove any fingerprints, dirt, grease, or oil. A small blast of WD-40 on the tip of the input shaft to help it slide into the bearing is OK. You don't want grease or oil in the clutch because it will hold the dirt and used friction materiel from the clutch and then become rock hard and interfere with it's operation. Often things not in a manual are left up to the mechanic's judgement and experience. I have also used high temperature brake grease (used to be used in old drum brakes where the shoes slide on the back plate) on the pivot points in the clutch like the clutch fork and the throwout bearing to prevent squeaks. Again, VERY sparingly. As for the pins in the output shafts, if they don't go in easily STOP, rotate the flange 180 degrees and try again. Are you using a ball bearing for the pilot bearing? The Citroen box has a bad habit of welding the input shaft into the crankshaft eventually.