The title says it all. I somehow managed to order an EL34 set online, instead of EL84, for my main amp (don't ask how) so I am wondering if I can use the EL34's in my backup Laney Cub 10 instead, which normally takes 6V6GT's.....I know the EL34's will fit in the 6V6 slot, but just wondering if I can use them straight and if there are any risks in doing so? (I am thoroughly ignorant of the electronics behind this)

It may very well blow the heater winding in the power transformer. And that's just the beginning of your risks.
Sell 'em on ebay or even better, see if the supplier can help you out; refund, exchange, etc.
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I would not recommend doing this yourself. The EL34 would draw much more current than EL84 or 6V6 ending up blowing or melting some components that would not be rated for the higher power developed by the EL34s.