CSS610 seems to be a tad too high to fit 1U. Brochure says height is 50mm while 1U net height is 44.45 (minus fraction of a mm for inter-device gap). Yes, it does come with rack mount ears, but will obviously eat into space below and above.
It also seems too wide to fit two horizontally in parallel into 19" rack: brochure declares width of 272mm (so two would need at least 544mm), net space between mounting posts in 19" rack is 450.85mm.
Compare CSS610 dimensions (272mmx50mm) to dimensions of RB5009, which was designed to be mounted 4 in a 1Unspace (in 2x2 matrix), which is 220mmx22mm.
When MT introduced RB5009, there were many expressing hopes for MT releasing some other device types (switches) in exactly the same form so that they could be mounted in same way as RB5009 can be. Ideally those other devices would stick to RB5009 external looks so tgat tgey would look well when mounted in same 2x2 matrix. We still hope to see this happen