I got the GPM mud tires:
http://mini-zracer.com/shop/product.php?pid=562
(tho i got the last 50 degree pair apparently
I dunno anything about the mini-z axle, but the tires fit on the mini-x rim. The only real issue is that the OL tires have a hollow space so that only 2 small edges touch the edges of the rim. I haven't run on it but I would guess the tires could slip off due to low surface area contact to hold them on. What I plan on doing is finding some kind of foam tape or something and cut it to the width of the gap on the inside of the OL tire and just tape it to the center of the mini-x rim to hold the tires on.
As for your second question, if you have the stock 5 cell pack, then it has no wires at all and the ESC has leads with battery springs attached that you slip in to the spring holders on the battery case to connect to the batteries. What I do when I'm not using it is pop the red battery spring out to disconnect the circuit. For charging, I still have to pull the batteries out, but I plan on putting a deans connector on the pack at some point. Anyway, you should be able to solder leads onto the pack, but you'd either have to solder them so you can still use the springs from the ESC leads or replace the ESC lead springs with something that will attach to the leads from the battery pack (ie. a deans plug). I haven't done it yet since one of the ESC leads is short and I'm not confident enough to solder on the PCB to add a longer lead to the ESC.
Sorry, lots of rambling but hopefully you can get enough info out of it :P
P.S. yea, the OL tires LOOK hot, and I'm guessing will RUN even better. Especially since I run on craptastic asphalt. Mmmm soft springs + OL tires...