I can help a little bit for Serbia and Montenegro.
- For first level of models for both countries can be similar, in different variation of colors. Both used heavy armored knights like:
And every other internet source about medieval Serbian army shows only hardcore heavy armored knight (Of course they were probably in minority, but used as elite fighters). There are no other more recognizable units than these.
- For second level: Serbia was under Ottomans, and there is a problem...
- Only units for this period can be Hajduks of Uskoks - they were something like guerrilla fighters for freedom. Also used "rented" for Venician and Austrian interest usually against Turks. Existed until fall of Ottoman empire. More info on wiki.
- Or Austro-Hungary irregulars. After fall of Serbia people started to migrate north in Hungary mostly, and they get rights to form irregular units to protect border:
Formed around 15 century. Light infantry with those hoodies.
Formed in later years from pandurs
And frajkors, also frontier fighters.
*For Montenegro: their independence was relative. They lived in tribal society, didn't have uniformed army, they also formed Hajduk and Uskok units. So same like Serbia with different color, and maybe some different details...
- Third: Here we find some differences
These are mostly uniforms from first and second Serbian uprisings (1804, 1815). My idea was to make for third level this revolutionary uniforms, and for the fourth level uniforms from Principality of Serbia (it's won't fit into game timeline, because Principality existed after... but until then Serbia actually didn't have organised army, and there is no other material for some other models. Only some improvised, for example to use some similar uniforms for that period like Austria or Russia or mix of those two, because of the influence of these countries they had or they would probably have if Serbia existed as sovereign country in game timeline)
For Montenegro is easier, these uniforms were used for very long time, and it could be used in different variation for last two levels.
- Serbia -
First picture - regular infantry uniform used in 1845. On second - two officers from same years. Third - some guardsmen. Fourth is horseman of Milos Obrenovic (Ruler) guard in 1835. And fifth is drummer uniform. It's not ideal but that's only my suggestion.
- Montenegro -
Little modernized version of previous level would be this one:
Hope I helped a little bit....