first run on my 18.5 WSM

Leaks are not an issue with the WSM.

Edit:  The WSM is not meant to be airtight.  Leaks will not affect the operation of the smoker.  However if the round is not true on the lid or body you may have excessive leaking and you'll want to contact Weber:

http://help.weber.com/

It will take several runs to get it broken in and seasoned.  It will settle down in temperature after a few cooks.  My WSMs ran hot for the first 3 or so and then settled in.

I wouldn't mix charcoal.  I'd stick with 1 brand and type (lump vs briquette) throughout each cook when possible.  There's certainly something to be said for trying different times but not for the same run.  Mixing it up will force your WSM to be inconsistent.  Think of it like throwing in 87, 89 and 93 octane along with some diesel and kerosene in your vehicle.  Yeah they all are fuels for vehicles but they react differently and I'm sure unpredictably when mixed and burned at the same time.

I cook with Kingsford Blue exclusively because I'm used to how it works in my smokers and grills.  Others have favorite brands and others prefer lump over briquettes.  At the end of the day its what you are used to using that is the best for you and your smoker.

You'll love this smoker.  I'd suggest checking out the minion method:

http://www.smokingmeatforums.com/search.php?search=minion+method&currenttab=Article

Finally, have fun.  

indyadmin1974 said:

It will take several runs to get it broken in and seasoned.  It will settle down in temperature after a few cooks.  My WSMs ran hot for the first 3 or so and then settled in.

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I had the same thing until I picked up a can of pam and sprayed the sides and lids liberally and fired it up for about an hour and re-sprayed it.  That will actually "season" the smoker so it doesn't run hot without having to go through a few smokes.