Ok ill try answer everything.
on the pebbles....imagine if you could swallow a football whole, that is what an axolotl could do make sure any rock you keep in the tank is much larger than there head if you look at a rock and have to think twice weather it is too small...it is too small, I can not stress enough make sure there is no physical possibility the rock can fit in there belly!
But if you ask my advice leave them out I know you say you are prepared to clean out often but trust me you will remove them in less than two weeks, If you are annoyed you bought them you could buy some aquarium sealant and make some caves, bridges, hides out of them, but again I would not bother.
As far as I know chloramines can only be treated with specific products. chlorine will evaporate chloramines will not. however, I do not believe chlorine will hurt the tank or filter just your Axolotl.
Like I say open up the filter and take a look. is it a proppelor and sponge or is it a canister type filter? I also think you should empty and refill the tank.
As long as you appreciate you MAY need to upgrade to a larger tank in the future (if they are aggresive etc) go and get them. There is someone on these forums 24hours a day and any advice you need will be here.
You have 4 days to ask everything you worry about on here and read everything you can, once you have a routine of feeding cleaning etc you will be fine and with us by your side you will be fine until then.
Refill your tank with de-chlorinator I use aqua plus but most are fine, stress coat is not recommended. add a block of frozen brine shrimp and the cycle will begin... get your sand (play sand from argos is good and is very cheap) and some more PVC pipes at least three. get some plastic plants for physical barriers and a few live plants to help with the cycle (if you ask the pet shop which plants are fine under lowlight and temperature, but java fern and moss are recommended) and your tank will be ready for them on saturday.
While they are in the tank you will be cycling the tank so you will have to do daily 20% water change a cycle takes between 4-8 weeks.
Have I answered everything?