You can skip the top fire basket bolts for sure. I imagine you could move them up a little along with the top ones so you could have 2 grates instead of one.
The bottom u bolts put your basket bottom at 6" from the bottom of the drum.... plenty of room to come down a little there thus giving you more leeway in positioning a second grate if you want to.
I can't give you any advice on the # of vent sliders though. I'm new to this drum also and don't have enough experience to give you an honest answer on it.
Weber 22 OTS/Smokenator_Weber Smokey Joe_Weber Genesis 330_Big Poppa UDS_ECB