In low DI at Weber St will it hurt me to have a SF with 70+ Ath and 15-30 RE?

what defense?

In a 3-2 or press not realy.
In man, a little.
In a 2-3, crippling.

Depending on what your opponent has, causes some good mismatches as well. 70ATH, but what are his LP and Speed?

TJ might be overstating it, but you would definitely want to be choosing 3-2 over 2-3 with that guy as SF. IMO, a 3-2 works best if you have a very strong center-type to play at both PF & C (great BLK, REB & ATH). If you have that, then playing a guard at SF is no big deal. It gets tricky if they guy you are playing at PF wouldn't be all that great at C. At that point, you are picking your poison.

So I have man D (sorry for not including that) and I'm just thinking a traditional guard type player. 70 ATH 80 SP 20 RE 20 SB 80 DE

He's going to cost you a few boards and SF's with 70+ Ath and LP are going to hurt you.

If he's a perimeter scorer and/or has high pass it might be worth it.