McCulloch claimed a hat-trick of headers - his first career treble - as the Light Blues romped to a 5-0 win over East Fife in their Scottish League One clash at Ibrox.
McCoist himself netted a staggering 28 hat-tricks during his own playing days with the club - but never netted three headers in one game.
He said: "It was terrific. He just told me that's his first hat-trick.
"I was surprised. I thought he had scored at least another one. It was a brilliant hat-trick.
"I actually thought it was a really, really good hat-trick and he thoroughly deserved it. He led the line well.
"I don't think I did score a hat-trick of headers.
"I remember I got two against (FC Alania) Vladikavkaz and I probably should have headed the third one in."
Nicky Clark set the ball rolling with a debut goal after just 30 seconds, while Lewis Macleod was also on the scoresheet.
Clark netted more than 40 goals for Queen of the South in the same division last season and McCoist said: "I was delighted for him.
"He has been showing enough in training to indicate that he will score goals.
"He plays on the shoulder, he runs through and he is extremely quick. I think he's got a good range of finishing."
He added: "It was a brilliant start. The wee fellow showed tremendous pace and awareness to get in and finish it.
"It was an unbelievable start after just 30 seconds. It was absolutely ideal. The level of finishing today was excellent."
Clark and goalkeeper Cammy Bell played as trialists, as Rangers wait for a transfer embargo to end at midnight after which time they can register their eight new signings.
McCoist said: "We've got the rest of the boys coming in and they can certainly strengthen the team and the squad.
"Outside the poor performance in the League Cup at Forfar, I think we've started reasonably well.
"But that's all it is - just a good start."
McCoist is keen to recruit one more player to bolster his squad.
He said: "There has been nothing since yesterday. I'm just going to see Craig (Mather, chief executive) just now and see if I can get one in."