Falkirk chairman Martin Ritchie accused Longmuir, the SFL chief executive, of being "irresponsible" by putting the plan forward on Tuesday – one day after SPL clubs set an April 15 date for their vote on plans for a merged league.
Their plan would have a 12-12-18 structure, the top two divisions splitting into three groups of eight after 22 games.
But Longmuir has raised the possibility of Rangers and Celtic reserve teams making it a 12-12-10-10 system.
He had argued Scottish football needed to safeguard itself for the possibility of the Glasgow teams moving elsewhere.
And he has responded to criticism of his idea, saying it was included in former First Minister Henry McLeish's review of Scottish football.
Longmuir said: "It may well be the case that league reconstruction won't happen before next season.
"But we have to get this right. Everyone has their agenda. I am looking at this objectively and now is the time to put ideas forward.
"All I am trying to do is stimulate debate. We are going through a huge round of deliberation."
Longmuir's timing has been questioned, with Raith chairman Turnbull Hutton saying: "This is a distraction.
"Colt teams were mooted in the Henry McLeish report. It's bizarre we're worrying about what happens when Rangers and Celtic depart Scotland.
"They've been going to depart the Scottish scene for the last decade. Where are they going to go?"