Let's compare Forte to a typical RB/WR player to an average starter at RB/WR. An Average Flex starter would be either RB #15 or WR #15 in a 12-team league (potentially 6 RBs and 6 WRs as flex starters divided by 2 = 3 + 12 = 15):
Forte: I'm projecting him at 223 points in that scoring system.
RB #15: Zac Stacy at 168.50
WR #15: Cordarrelle Patterson at 148
This means that RBs are a lot more valuable in your flex than a WR since this is a Non-PPR, unless you can get 2 amazing WRs.
So, that gives Forte an advantage of about 55 points over the #15 RB, or 75 points over the #15 WR as your flex.
Manning: I'm projecting him at 487 points in that scoring system (6-pt passing TDs). An average starter in a 12-team league would be the #6 or #7 QB.
#6 QB: Nick Foles at 411.45 points
So, 487-411 = 76 points.
That pushes me towards Peyton, since there's less of a difference if you start a RB there. I also don't see much of a point difference between Zac Stacy at #15 and Rashad Jennings at #21 in that scoring system (I have Stacy at 168 points, and Jennings at 161 points), which means there's a whole lot of RBs that you can grab to be your flex that would still lean towards Manning.
However, the difference between the #6 QB and the #12 QB (Romo) is 29 points.
So, ideal would be to keep Peyton, and grab a decent RB2 to fill that flex spot.
My draft plan here would be to keep Jamaal and Peyton, add the best WR that you can for your WR spot, and then make sure you get a decent RB2 to fill that flex position. Doesn't have to be great but does need to be decent.