Because I'm a moron, I've gone and ordered a matched pair of JJ EL34s and only 3 JJ 12AX7s when my amp actually takes 4.

My problem is this: I'm impatient as hell and I can go into town now to pick up an Electro Harmonics 12AX7. Would there be any problems with chucking this in there or should I just order another JJ online?
You can mix and match pre amp tubes as long as you are using the right tube type for your amp.
You can try putting the 12AT7 as phase inverter tube. There will be change in sound but some people actually like it. Some amps sound cleaner that way. 12AT7 is compatible with 12AX7 so you can put it anywhere really but it has less gain. In phase inverter spot the loss of gain is least noticeable and as I said, it may make sound cleaner.

ESP LTD F-50 + Tonezone
Cort EVL-Z4 + X2N
Cort EVL-K47B

Marshall Valvestate 8100
Randall RG1503
Bugera 333
Peavey Rockmaster preamp

Line6 Pod X3