Just to answer some things mentioned above, you can still encounter the Mana Surge Gnomes in a Gnome-a-Palooza, but the chance is much lower than before.
They have the same encounter rate outside of the Gnome-a-Palooza as before, this has not been changed. You have a chance of encountering the Mana Surge Gnomes in every place you can find Gnomes. (Unlike Pet Gnomes they don’t only appear in PVP and Arena.)
Lastly, the Mana Surge members have the same encounter chance. If you are seeing more of one than another this is due to RNG.