Rebel:

Tapcon screws are unique in that they can be driven into a pilot hole drilled into concrete without using a plastic or lead anchor to hold the screw.

But, for your purpose, I wouldn't use them. They're ordinary steel, and in that application they'd rust out in a few years so that you'd never be able to get that toilet flange off if you needed to.

If I were me, I would:
a. drill pilot holes for the toilet flange to a depth of about 1 1/2 inches
b. enlarge those holes with a 5/16 inch masonry bit and vaccuum the dust out of the holes by inserting a drinking straw in the suction hose of a vaccuum cleaner.
c. insert 5/16 inch diameter by 1 1/2 inch long lead anchors into the holes so that the tops of the lead anchors stick out a little bit from the concrete floor.
d. Use a counter sinking drill bit to cut the tops off the lead anchors so they don't protrude above the concrete. Counter sinking bits are typically used on wood so that the heads of flat head screws are flush with the surrounding wood. But, lead is such a soft metal that the counter sinking bit won't have any problem countersinking lead.
e. Screw your floor flange down with STAINLESS STEEL flat head size # 10 or # 12 sheet metal screws. (See PS below.)

You should be able to find 5/16 inch diameter by 1 1/2 inch long lead anchors at any hardware store or home center. Ditto for the stainless steel screws and ditto for the counter sinking bit.

If you're not following this game plan, Google "5/16 inch lead anchors" and "countersinking bit" to see what they look like, and it should all become apparant to you.

This way, neither the lead nor the stainless steel are going to corrode so you don't have to worry about facing real difficulties if you ever crack that floor flange and need to replace it. With badly rusted ordinary steel screws holding it down, you might have a real fight on your hands getting those old steel screws out. With stainless steel screws in lead anchors, you'll not have any problems replacing the floor flange in future if necessary.

PS:
The difference between a wood screw and a sheet metal screw is that the wood screw will only be threaded over 2/3 of it's overall length. Sheet metal screws are threaded right up to the head of the screw. Most handymen use sheet metal screws for just about everything and rarely, if ever, use wood screws. The name "sheet metal screw" gives a mental image of being small, but sheet metal screws come in all the same sizes that wood screws do.