QUOTE (apple @ Apr 18 2015, 05:40 AM)

SR4: the Ally Spirit has its own spirit type and is not an upgraded "former tradition spirit". Nothing in the SM indicates that you upgrade a previous spirit. Before conjuring an ally, the initiate must fi rst prepare a unique spirit formula which shapes the ally spirit’s essence and dictates its abilities and powers There is no further reference to any other spirits, just that you need to create a unique formula.
However, You must use a Bound Spirit, and since it is not an Ally before you perform the ritual, it obviously must be a spirit you can summon, yes? You Summon a Spirit Normally, following Normal Rules, Then you Bind the Spirit, Again using normal rules. THEN you apply the unique formula to transform the spirit into an Ally Spirit (paying Karma to craft the Ally Spirit in the formula's image). Since you Must summon before you can do anything else, the spirit MUST, by definition, be a spirit you can summon normally.
QUOTE (Ally Conjuration Ritual, Page 104, Street Magic)
The initiate first summons the ally at the desired Force, following normal summoning rules (p. 179, SR4). Immediately after summoning, the ally spirit must be bound following the normal rules for binding (p. 180, SR4). Drain is calculated as if summoning and binding a normal spirit of equal Force.
I would say that it is pretty cut and dried (The caveat of requiring normal Summoning and Binding rules tells me that you cannot summon a spirit you are normally incapable of summoning as the basis for your Ally). Yes, You need a unique formula, but you must first summon the ally as a Normal Spirit, and then bind it as a normal spirit.