We started a dynasty league with IDP last year and i think it was the most interesting league i was in. We had 10 team - 2 QB, 2 RB, 3 WR, 1 TE, 1 Flex, 2 DL, 2 LB, 2 DB - 25 bench, 5 IR, 5 Taxi. For the upcoming year we voted to add an IDP Flex (most likely will always be filled with a LB but it allows for flexibility) with no roster expansion. The main reason I was for it was, to make IDP a little harder from a waiver wire standpoint cuz even in our league you could still stream it if you so desired and increase offensive depth on the waiver wire.
I think the reason most say to add more then 1 IDP is due to the fact that it’s not a challenge if everyone has an elite IDP player - it really doesnt take any IDP knowledge to get 1 player. I would recommend that if you add IDP to you league to add more more then 1 position - thinking at least 3 LBs or 2 LBs and a flex… somthing like that. i think you could do the 2 DL, 2 LB, 2 DB pretty easy and it really adds a fun dynamic to the league. I would recommend a scoring something in the range of Blocked kicks - 6, Forced Fumbles - 4, Fumble Recovered - 4, INT - 6, Kickoff returned TD - 6, Passes Defended - 1.5, Punt Return Touchdown - 6, Sacks - 2, Safeties - 10, TkA - .75, TFL 1.5, TkS 1.5, TD defense - 6. However, in our league the if a player gets a sack they get the 2 for the sack, 1.5 TkS/.75 TkA, & 1.5 TFL so a sack gets you about 5 total points.