The lee collet die has enough room in the shoulder to accomidate the AI configuration. You just need the stock, 25-06 die.
of course... these products are avalable from a number of sources as well...
As for the extra mandrels, just call their customer service(Lisa if memory serves me...), and tell them you want a couple of standard diameter manrdels, and a couple of undersized mandrels. Stock mandrels will usually net .001", undersized will get you .002", but a little polishing will get you any diameter you want. Chuck it in a drill, and polish with 600grit to the diameter, then finish with 1500 or 2000 and it will be super smooth.
If you don't have the stuff on hand you can send them here and ill do them for you gratis.
I usually do one at .001", .0015", .002", and .003". Occasionally ill order one larger... then polish it down to .0005", but thats usually only my tactic for soft seating VLDs... and even then, .001" usually does it.
Ill also occasionally snip the decapping pin from the mandrel to allow me to size primed brass... comes in handy when you want to experiment with different neck tensions loading at the range. Full prep on the brass, prime, and then size and seat at the range... it also lets em know if the primer pockets are too loose earlier in the batching process so I haven't invested a bunch of time trimming, chamfering, and otherwise processing cases that have bad primer pockets.
Oh, and one more thing... you can also use machine bushings between the shellholder and collet to controll the amount of the neck you want to size. Some like to do that loading... but it also comes in handy in removeing donuts from the neck... finding length for ogive contact to the lands... and a few others.