In 2012, his local Pumas team called him up to represent them at the Under-16 Grant Khomo Week tournament, where Nkosi made three appearances.

He scored a try in the unofficial final of the 2013 tournament in Polokwane, but his side fell short, losing 29–45 to Western Province.

[17] He was released from the South Africa Under-20s training camp to play two matches for the Sharks XV in the 2016 Currie Cup qualification series – playing off the bench in a 48–18 victory over Namibian side the Welwitschias in his first class debut[18] and starting their 24–16 victory over Western Province a fortnight later[19] – before being included in the final South Africa Under-20 squad for the 2016 World Rugby Under 20 Championship tournament to be held in Manchester, England.

[20] He started in their opening match in Pool C of the tournament as South Africa came from behind to beat Japan 59–19.

Nkosi suffered a thigh injury which ruled him out of the remainder of the competition,[24] so he didn't feature in the semi-final – where they faced three-time champions England, with the hosts proved too strong, knocking South Africa out of the competition with a 39–17 victory[25] – or their final match, the third-place play-off against Argentina.