From what I have read they found that hoses which had already had oil in them from R-12 would hold 134a without leaking... but new hoses would need to be the barrier type...
So I think if you just flush well both directions... you can recycle the first flush, then end up with new flush last on each item... you should be fine. Blow dry as best you can and it is nice to do this very close to putting the system together... at the very least after drying as best as you can cover all open ends until you put it together. Just like you would treat the new reciever/dryer....keep closed until the last possible second.