There are only 9 buildings allowed per building tree. In the Camilian/Polybian MIC tree there is a level 1 barracks then 4 Camilian and 4 Polybian 2-4th level MICs. In order to have another era, another building tree is needed. Since you can't upgrade from one tree to a different tree, the script places Marian MICs and damages Polybian MICs. Just tear down all of your Polybian MICs.

Rome is a faction that used heavy infantry. They didn't use a whole lot of cavalry or archers. There is no 'Roman' cavalry or archers in the Marian era. There are a few auxilia cavalry units in Gaul, Iberia, Germania, and Thrace. For the most part the Romans had to use allied (regional) missile and cavalry units when they needed them.

Not all regions have cavalry. Egypt is one of these regions. Do not expect all kinds of units from everywhere. The EB system is based on what kinds of units that historically a faction used from an area, what they would have used, or what type of natives would be willing to work for a faction. This leaves some places will little to no units to recruit.

And welcome!