Buildings are easy. Many buildings sold as Spanish or Italian Napoleonic Wars buildings will do. You can mix in a lot of boxy flat-roof North African buildings too, to model standard puebla construction. Spanish and Mexican architecture also has a lot more arches than buildings sold as "North African", but you can add some with a grinder or putty.
Standard bushy plants and trees are easy to make out of standard model railroading supplies – tufts, ground foams, clump foam, tree armatures, etc. Spend time virtually driving around the countryside in areas where battles were fought to get a sense of the green/brown mix and prevalence of bare dirt and rock. Mexico is big, and home to a lot of varied environments, but all the lush green jungles are down south where the war didn't go (much). The central and northern coasts are greener than the higher altitude central areas and northwestern deserts, but still tend toward brown grass/green trees like the US southwestern states.
Palm trees are hard to find small enough. Look for Z scale palm trees on eBay, and good luck. I did this search in 2018-2020 and all the smallest palm trees I could find were still oversize for 6mm scale.
Cacti for the desert areas may be impossible to find in 6mm scale, but maybe at that scale it doesn't matter much.
- Ix
gets started with WWI British in 15mm.
