Most people have full human rosters, and will favor backups on special teams (to save starter energy)
To preserve energy I usually spread the positions around the roster. You may have already seen this, but these charts will really help:
KRS (Kickoff Return Unit)
-------------------------
9__7___5__4__6___8__10
______________________
_______1__2__3________
______________________
______________________
__________KR__________
PRS (Punt Return Unit- also from KRS)
-------------------------
9____5_3_1_2_4_6____10
______________________
________7_____________
___________8__________
______________________
______________________
_________P____________
I'm just going to go through the KOS for now, but watching replays you should be able to figure out where you need the fast guys, where you need strong guys, etc.
1, 2, 3: These guys are the back 3 in KRS, and make up the middle in PR. They need to be good, strong blockers, but for KR they really need some vision/agility/speed to pick up key blocks for the kick returner. Good options are FB, TE, OT.
4, 5, 6: These guys are heavy-duty linemen on both PR, KR. Good spot to put Centers, Guards who don't need to move much and will hold their blocks.
7,8: These guys are important most on PR. They are in the best position to take on runaway gunners and fast tacklers- high vision, agility, speed are essential, without being too weak. Fast FBs, TEs can work, WRs who won't get run over. This is also a decent spot to put a LB or SS who has the movement skill set, along with some strength to make the blocks.
9,10: Due to the KR/PR pathing, these guys are largely useless. Theoretically they should pick up the gunners in PR, but they always run to the sidelines first. In KR, they don't even need to be good blockers since the KR rarely runs to the sides, and a KR caught from a tackler in those positions has usually already made a 40+ yard return. Suggested options: WR, backup QB, garbage builds. If you want to give them any chance to get a block, speed will be necessary.