Stefanos Tzimas also featured as the under-21s beat West Brom 4-0 in their opening PL2 game of the season.
Both Greek forwards went on for the last half-hour at Lancing, watched by first team head coach Fabian Hurzeler.
Kostoulas signed for almost £30 million in the summer from Olympiacos but has had to adapt to new physical challenges.
He ended last season sidelined by injury and has only started training with the Albion squad in the last ten days.
Tzimas had a brief outing in a behind-closed-doors friendly with Wolfsburg last Saturday.
Shane Nti, who scored the winner at Aldershot in the National League Cup in midweek, completed a hat-trick against the Baggies.
His third was from a penalty forced by Tzimas. Callum Mackley was also on target with Kostoulas involved.
Centre-back Freddie Simmonds, who went off early in the Aldershot game, was fit to play today.
Albion: Ramming; Mackley, Simmonds (Slater 78), Atom; Tasker, Albarus (Owusu 46), Robertson (Oriola 71), Keogh; Howell (Kostoulas 61), Belmont (Tzimas 61), Nti.