I just poked around the web a bit. At MLW, the 170 requires 4,177 feet for landing. 17/35 is 5,460 feet, and 8/26 is an even 5,000 feet. Interestingly, even the 145 only requires 4,593 feet for landing.
The issue is takeoff, not landing. The 170 needs only 5,217 feet for takeoff at MGTW. The 145 requires 7,448 feet.
The CRJ700 has similar airfield performance numbers to the 170, though the 700ER requires just slightly too much for takeoff at MGTW from 17/35.
So, the 170 and 700 (but not 700ER at MGTW) can use 17/35 for both takeoff and landing. All RJs can use any runway at PHL for landing.
The issue is takeoff, not landing. The 170 needs only 5,217 feet for takeoff at MGTW. The 145 requires 7,448 feet.
The CRJ700 has similar airfield performance numbers to the 170, though the 700ER requires just slightly too much for takeoff at MGTW from 17/35.
So, the 170 and 700 (but not 700ER at MGTW) can use 17/35 for both takeoff and landing. All RJs can use any runway at PHL for landing.