actually, you do not NEED magical guard as a power for an ally spirit, as he may aid sorcery as one of his services, adding his force to any sorcery (casting & counterspelling) test of the summoner (see aid sorcery SR4A p. 187) for (or against) any spell.
Which IMO is even better than magical guard, as overlapping counterspelling is handled like an assisted test (meaning you would only roll an additional number of dice on your counterspelling test equal to the number of hits of your ally's counterspelling test up to your rank in counterspelling).