Please be careful with this material, because it is severely tainted by nationalist agendas and we know where that finished up just a few years ago. The various nationalities in the Balkans claim all kinds of units and events to push their own national identities and those very agendas mean they do not read the Imperial documentation, consequently understanding the background even less. I have seen one Romanian site claiming IR51 Splenyi "Legion infernale" as essentially Romanian troops, simply because their recruiting area was Transylvania, which has been part of Romania since 1920. That ignores population movements, the German colonies (which supplied many officers), the Hungarian and Szeckler populations (pushed out since 1920) and the fact that Romanian troops were considered to be among the worst in the army during the Napoleonic period!
If you look at the Wiki entry, you can see this https://en.wikipedia.org/wiki/Serbian_Free_Corps
The author is talking about various Serb warbands in the revolts against Turkish rule, but we find the Seressaner (actually a police unit in the Licca regiment of the Karlstadt district of the Military Frontier since 1754) and the "St Georg Freikorps from Croatia" (Warasdin St. Georg was the 2nd Warasdin District regiment). Quite what the difference between a fusilier and a musketeer is defeats me, but the source for that appears to be the same Serb text (of 1866 apparently) as Orange mentions and there they are 4 companies of Sharpshooters. That of course is actually a reference to the Sharpshooter detachments with each Grenzer regiment! The Warasdin district was about 50/50 Serb and Croat in population.
As I mentioned, the Wiki article is talking about the various Serb warbands, which sprang up in Bosnia and Serbia (then both in the Turkish Empire) when Austria and Russia attacked in 1788. The war ended in stalemate, but both these militias and many refugees were taken into the Frontier area and from there, the Serb Freikorps was formed within the Imperial army. They were increased with the Wurmser Freikorps (the infamous Red Mantles) in 1793 as the Grenzers had taken heavy casualties in the Turkish wars and could not supply many troops when fighting started in the Netherlands in 1792. The same happened after the Serb rebellion of 1804 was finally suppressed by the Turks in 1813 and there is a Serb Freikorps for the War of Liberation of 1813-14. The Serb tradition was that a warband was led by a Voivode, whose status depended on the size of his warband, but when they were absorbed in to the Imperial army, as the official annual Schematis shows, there was just one Serb Freikorps, although it was large enough even in 1798 to form two of the short-lived regular Light Infantry battalions. So, as was the case with all these light/freikorps units, they are named after their commanding officer at a particular time, hence Major Branovacky was commanding them at Neerwinden, as their usual commander, Mihailovich was commanding the composite larger formation.
On reflection, I think the colourful "Grenzer NCO at Mantua" in the russian book is actually a Seresssaner Harambassa (sergeant) from 1816.