You will have more problem if you delete any of your gigs because as a new seller/level 0 seller, according to new level system, you can have only 4 active gigs. So you won't be able to create anymore gigs. So better to keep those gigs.
As for Milestones: Why don't you try to send a Custom Offer to your buyers? That way, you will have Milestones option while setting an offer.
Also, your reviews won't be removed no matter how many times you do changes in your gig even if you delete them.