You want a flange with a SS metal ring so that it has both more strength, and won't rust. This also means you need to use SS or brass closet bolts to hold the toilet. The flange ring needs to be anchored into the slab. Lead anchors or tapcon screws can work if the slab is solid there. The toilet MUST NOT rock on the tile! Add shims, if necessary to prevent it from moving. If the toilet does not rock, and the flange is at the design height (optimally, installed on top of the finished floor, or equivalent), it should squish the wax when you set it and not leak. Since the tile can be slippery, and to prevent crud from accumulating underneath the toilet's base, use caulk around the front (leave the back open). This also will keep the thing from moving and breaking the wax seal. Polyseamseal is good for this, since it is easier to lift the toilet, when that becomes necessary - silicon would have to be laboriously cut through to release the toilet.