Pros & Cons of combining RAM

An old 2008 system has this fine 2x4B = 8GB RAM set in it:

http://www.kingston.com/us/memory/search/discontinuedmodels?partId=KHX1600C9D3T1K2/8G





If this additional 3x1GB set is available:

http://www.gskill.com/en/product/f3-10666cl9t-3gbnq






What are the pros and cons of combining those two sets, since the old Asus P6T Deluxe motherboard has six RAM slots.
Can it even be done since the voltages may be different? The end result would be 11 GB of RAM instead of 8GB, what are the practical reasons against this, even if it could be combined?

Moved from Classifieds "Storage and Memory"

Those two setts have different Speeds I would not combine them. You can take a chance they might or might not work.

Board will not boot with 5 sticks. RTFM
IMO, no point. Stick with your 8 gig in dual channel or buy the correct triple channel kits you want.
Do it right the first time and avoid issues.

I agree with Mr. Scott.
Like birds of a feather flock together, install like ram together and you will have no issues later.