Installing 1"x2" porcelain tile on kitchen wall

What is the proper adhesive brand and trowel that should be used for installing 1"x2" small porcelain tile (glazed & crackled) on a kitchen wall? I heard that when installing small tile mats, one problem is sagging. How is this prevented? The wall surface is Hardibacker screwed onto painted drywall. The tile area is under cabinets. I plan on using an epoxy grout called Spectra Lock purchased at Lowes. It was also recommended that I seal crackled tile.

Use a 1/8" or 3/16" square notch trowel with thinset, the mats won't sag. If they do for some odd reason use spacers the same size as the spacing already in the mats. If the mats are basically all field tiles with no pattern then also stagger the joints.