Heffernan arrived from Kilmarnock the day before the Scottish transfer window closed and cancelled out Stevie May's 17th-minute opener with a tap in from eight yards 10 minutes from half-time.
Strike partner Collins was brought north from Swindon in the summer by Fenlon for £200,000 and finally broke his duck for his new side with a hooked volley in the 59th minute which was enough to ensure the three points headed back to the capital.
And boss Fenlon believes both players can form a fearsome partnership up front for Hibernian.
He said: "It will be a boost to their confidence. Strikers live and breathe on scoring goals and you can do all the work and have a decent game but when you come off as a striker you're disappointed if you haven't scored.
"I didn't think James' was going to get over the line to be honest with you as he had one of those days and I thought Alan Mannus was going to get back to save it but I'm delighted for him.
"What Paul gives us with Rowan Vine and James is someone who comes in and links the play.
"And particularly when you've got midfielders who want to get it into him early when he gets into good positions but he's also a very good finisher.
"We've got some decent forwards at the club now so hopefully they'll keep going now."
St Johnstone manager Tommy Wright refused to blame goalkeeper Alan Mannus for Collins' winner which crept over the line after the Northern Irishman got a hand to the strike.
He said: "Alan wasn't at fault. He was excellent and done well and Alan isn't the reason why we lost the game.
"For their first goal we really should have had a free-kick but Murray Davidson didn't go to ground.
"The second goal was a poor goal from our point of view and how we conceded it.
"But we started both halves really well although we didn't make enough of the opportunities we had."