It is certainly not necessary to use a long arm on the Verdier, but a 12" arm can easily fit on this table. The general issue of 9" vs. 12" arms has been discussed frequently. Do a search for the pros and cons. All I can say is that the 12" wand on the Moerch DP-6 is supposed to sound better than the shorter wand. I haven't done the comparison myself. I just went with a known winner. Actually I first heard the 12" Moerch at my dealer on a Verdier. This is his standard setup for the PV.
The longer wand on the DP-6 definitely does not lock you into low compliance cartridges. The Moerch wands are available in different masses so you can match the right wand to your cartridge. The 12" Red is the medium mass version that seems to mate well with a wide variety of cartridges. There is a Blue wand that has more mass and would be better with low compliance models. I would think the Red would do fine with your Shelter 901.
Good luck on your selection!
Dave