Early on, yes, to some extent, though while that practice was in use the numbers of the standing army were not very large. However the devshirme became less and less of an issue as time went on. The devshirme was a potential gateway to status and power and increasingly Muslim families began to compete to get their children admitted, “smuggling” them in if necessary ( this also occurred with Christian families - we have examples going both ways, with some parents attempting to bribe recruiters not to take their children and others bribing them to take those that didn’t meet recruitment standards ). As early as 1515 1,000 Bosnian Muslim boys were recruited into the corps and by the time of Suleiman I’s death in the mid-16th century at the height of Ottoman power the the devshirme was a declining practice. By the mid-17th it was essentially defunct. Thereafter Janissaries were overwhelmingly of free born Muslim origins, recruited normally, though many were still European ( Albanians and Bosnians in particular, also the source of large numbers of semi-professional irregulars ).
Meanwhile Ottoman rule continued in places like Bulgaria for another 200 years beyond that. So it was not really that Ottomans were discouraging conversion to keep themselves supplied with military slaves. They never faced the possibility of even a hint of such a shortage and eventually they no longer even made use of them. It was other, local factors that encouraged or discouraged conversion in the Balkans.
True, although there was a famous Basque Muladi family that ruled in parts of what’s now Southern Navarre and La Rioja, though, called the Banu Qasi, and there were a bunch of other native Spanish dynasties/vassals of the Umayyids, as well, most notably the Banu Tujibi (rulers of Saragosa) and Banu al-Tawil (rulers of Huesca).
Isn’t the main difference between Bosnians and Serbians (ethnically speaking) that the former are Muslim while the latter are Orthodox Christians? So would it be accurate to say that what happened is that those South Slavs who converted to Islam became Bosnians, while those who didn’t became Serbians?
Croats, as I understand it, are Catholics; I believe this is because they were under Austrian domination.
More or less. Bosnian was a geographic descriptor, whereas Serb and Croat were tribal - but all were linguistically approximately the same folk. Bosnia did exist as a distinct state prior to the Ottoman conquest, but it was something of a bud off both Serbian and Croatian polities ( and was variously disputed by both ).
It’s real distinction started with the now defunct Bosnian church, often accused of being heretical and dualist. It has been speculated that persecution of the native church may have had some linkage to the ready acceptance of Islam in the area, but it is all rather murky.
ETA:
Actually Croat Catholicism started long before the Austrians entered the stage. Croatia was the first south Slavic state to really organize itself as such and it just happened to convert to Catholicism when it did.
One study (New York Times article) in 2008 found that 31% of the population of Spain and Portugal is descended from Jews or Moors. An awful lot of people converted and stayed, and their descendents are there to this day.