This is a physical and environmental question more than anything fancy.
We have a site with a small data rack and until their old hardware is stripped out to make space, we are looking to stand an MS-120-8 and 2 x MX64's on top of each other. The rack is in an A/C controlled room so there's a reasonable supply of cooling available.
Is that likely to cause any issues with temperature or performance in the short term?
Agree with previous posts, you're in the Meraki green, no worries! Just wanted to add one thought. We have had customers with small stacks of 3 or 4 pieces of equipment like that right on top of one another, but NOT in an air-conditioned or even well ventilated room. For US$8 they purchased a USB flexible fan stick and plugged it into the USB port of the MX64, and coiled/pointed it to pass air between the equipment. Worked great, and the before/after was a difference of over 30 degrees F.