Sorry I just looked at maxijet and it looks more like you need 1/2" ID tubing that can slip over the nozzle. J&L has that:
http://www.jlaquatics.com/phpstore/s...t_ID=pl-tub012
Unfortunately until you have the items in hand it's hard to imagine up a wiggle-room-free shopping list of items for you. If you buy a kit from BRS I'm sure they'll send you everything you need. If you source your own parts (I think I've seen replacement canisters for RO filters at HD running around $20-30ish but) then you might need to get creative a little bit.
As for stepping down the 1/2" to 1/4" you can should be able find adapters for that. The HD nearest to me has a surprisingly good selection of John Guest style pushfittings ranging in size from 3/4" down to 1/4" and various combinations thereof.
Or you can get creative. One time I got fed up looking for the bits I needed in my junk plumbing drawer so I just took some 1/2" vinyl tubing, stuck the 1/4" tubing into that and then just used silicone in the gap to hold them together. Ghetto FTW!
It never leaked for me.
Hope this helps..