Is it possible to make the Imperial/Stormcloack questline AFTER you finish the peace council and Main questline?

Probably.

ya i did on my nord warrior infact i joined and won the civil war after that quest lol

So is it safe to finish the mainquestline before the imperial/stormcloack questline? I remember Tullius saying that the treaty was until the dragonborn could finish the dragon menace.

there is nothing baring you from doing the quests you can be a badguy and break the treaty if you want

You can but the questline will not progress to take Whiterun or defend it until you complete the MQ to my knowledge. Balgruuf will not talk to you about Ulfric's axe.

The only time I've ever finished the civil war quest line was after I'd already done the main quest. Although on my current character, I might do the civil war quest line before the MQ.