Can Full Impact allow for flex players?

Yes. Full Impact is flex position friendly.

Let's say your league starts 2-RB and 2-WR and one flex (WR or RB).

Just tell Full Impact you start 2.5-RB and 2.5-WR.

Now, if you know guys typically start more RBs than WR at the flex position, then adjust accordingly - example: enter 2.8-RB and 2.2 WR. This will only further sharpen your edge!