Can I take the jobs from Delvin and Vex and get the unique jobs before finishing the thieves' guild main questline all the way?

I'm at the point where I need to return the skeleton key to the twilight sepulcher but it's easier to do all of these quests while I have it in my possession.

They keep giving me jobs but it's taking forever for the special quests to come.

It's up to you when you want to return the key, some people just kept it.