It's usually best to keep all Q's in one thread, but I understand why you started a new one in this case, no problem.
You're using Mapei Silver, (hopefully sanded), most people would select Mapei caulk too. But, Mapei caulk does not meet ASTM C920 specs for this purpose.
Laticrete's Latasil is good stuff and there should be a color that is very close. Being 100% silicone may make it harder to work with for many people, but if you're comfy, OK. The slight difference in color may not be noticeable, considering that grout/caulk within the same brand vary in color anyway.
Latasil meets ASTN C920 for many uses, only does not meet "Use T" which is for use in joints subjected to pedestrian and vehicle traffic. (Did I mention Laticrete makes great grouts too?). But since the joints will be (supposed to be), 3/8-1/2" and slightly below the surface it should be ok. It's better than nothing and can be redone easily if necessary.
2) I broke the tile pattern into four 10ft x 10ft sections (slab=10ft x 40ft total); and seperated these sections with a 1/4in grout line. The caulking will go into these 1/4in joints, for expansion purposes, in addition to the perimeters.
Well, you know 1/4" is less than industry recommendations, but do as you like. Problem is that if this job goes bad in a couple of years the HO will do research and find ways to make you pay.
3) Laticrete Latasil caulking reads very good for this application but the local distributor only has 4 tubes....
Need more? No problem.