In general, hard-setting adhesives are better than pressure-sensitive adhesives, but ATG is among the most costly and least dependable adhesives used in frame shops. For all pressure-sensitive adhesives, "pressure" is the operative term, so ATG probably will fail if it is not thoroughly burnished, and it can flow over time in any case. Ordinary frame glue is OK for general purpose framing, but for preservation framing, I suggest using an acid-free PVA or EVA glue. Dabs of inexpensive acrylic gel medium also are suitable for joining mats in levels of all framing, but may take longer to dry.
Yes, expansion/contraction cycles are quite normal in controlled environments, but they increase with radical changes of moisture content and temperature inside the frame. That is why completely closing the frame with glazing and a tight-fitting dustcover is important, in order to slow down the rate of change inside the frame. As tedh noted, 1/8 is the usual allowance for normal expansion and contraction cycles, and very large frames might require more.
Also, be careful not to fit the frame too tightly front-to-back. That is, do not push down when installing the fitting points, because the front-to-back pressure of too-tight fitting also can prevent normal expansion/contraction cycles. For example, in sectional metal frames fitted with spring clips, have you ever noticed the mat' edges ruffling under the clips? That is due to the pressure of the clips preventing expansion/contraction.
After fitting, the typical framing package should be loose enough that it shifts a little if you shake the frame side-to-side. That will normally (and properly) make the glass rattle, and in my shop, if you can't rattle the glass after fitting, then it's too tight. [Exception: Acrylic DCO framing]