OmnipureQ5605 and (2) Q5633 Replacement Sediment Carbon Filter Cartridge Set

Our system only has a prefilter ,postfilter and membrane. Is this the replacement we need?

I'm about to replace these myself for the first time. what size and material of o-ring should i use when i replace my filters?

I currently have a q5605 sentiment pre filter and 2 q5621 carbon filters. can i use the q5633 instead of the q5621?

We have a Purotwist 4000 unit with only 3 tubes is this the set we need?