1)
Where was Sha'ul at this time?
1.
Radak, Malbim: He went with Shmuel to Giv'as Binyamin, and counted the men there. 1
2.
Kli Yakar (Lani'ado) #1: He was still in Gilgal. Shmuel did not want to join with Sha'ul, after Sha'ul did not fulfill his command. Shmuel went to Giv'as Binyamin; this is not Geva (also Radak says so), to which Sha'ul went.
3.
Kli Yakar #2: He went with Shmuel to Giv'as Binyamin, counted the men, and after this, Sha'ul and Yonason went to Geva and joined with other Yisraelim.
2)
Were all 600 men from the 2,000 who were initially with Sha'ul?
1.
Radak: Yes.
2.
Malbim: No. Some were initially with Yonason, who was in Giv'as Binyamin (verse 2).