Was this groundswell a risk which now replaces half of a two-party system, or do they use ballots which make third-party support safe?

Serbia uses a proportional "closed list" electoral system, which essentially means that a party internally chooses its representatives, and when you go to the ballot box, you vote for a party, rather than an individual representative. and the fact that it's proportional, rather than majoritarian (first past the post/winner takes all), means that third parties are more viable. But historically those third parties have been small, ethic minority-based parties.

According to Wikipedia: "Eligible voters vote for electoral lists, on which the registered candidates are present. A maximum of 250 candidates could be present on a single electoral list. An electoral list could be only submitted by a registered political party. To submit an electoral list, at least 10,000 valid signatures must be collected, though ethnic minority parties only need to collect 5,000 signature to qualify on ballot. At least 40 percent of candidates on electoral lists must be female."

The requirements here are fairly strict, and prior to the student list, there wasn't really a traditional opposition bloc. One likely struggled to cohere in the past, because of those requirements.

The movement to get the Student List on the ballot in the first place came out of a larger, non-electoral movement of protests and strikes against corruption, which provided a basis upon which a coherent opposition party could come together, and they could overcome signature requirements and the like.