Hi,
Shadow Spectres use the fast attack slot on the force org chart. With your allies chart you only have one fast attack slot. As to there effectiveness, they aren't too bad. They can do all three areas to varying degrees of success, either anti-tank at 36 inches ( st9, ap3), MEQ hunting at (st6,ap3) or horde (st4,ap5 blast) from memory as I don't have the book handy. The latter two are there standard shooting options, while the former is when they use there ghost light and fire through the Exarch. Hope this helps.
Cheers,
Nyos