1. Background Count. When you use a magical skill inside a background count, it's a penalty to your dice pool. When a pre-existing magical spell, spirit, or focus enters a Background Count from outside, it's a reduction in Force. A skill limit based on Force (i.e. spellcasting, summoning) applies when and where the skill is used. If a spell or spirit later enters a Background Count and has its Force reduced, that doesn't retroactively change the limit for the test. You still keep all the hits.
2. Wards. My personal rule of thumb: Anyplace that can afford full-time security guards can afford wards. The rating of the ward corresponds to the number of guards on the night shift (up to about rating 5), unless there's a good, specific reason for a higher rating.
Figure a typical mid-size business can afford to hire a freelance magician ("professional" skill rating 6, average Magic attribute 3) to come around at regular intervals and ward the place. Working solo, her Ritual Spellcasting dice pool is 9. If she sets up a rating 3 ward, then the ritual takes 3 hours and averages one net hit, meaning she has to come back and renew the ward every week. Say she makes it a regular weekly appointment, charges 100 nuyen per hour plus ritual materials, and handles five or ten clients per week. Not a bad living, leaves plenty of free time. And if anyone messes with the ward, she calls Knight Errant to report an astral break-in.
Bigger companies can afford better magical security firms, who'll have ritual teams set up higher-rating wards for a lot more money, and might agree to investigate disturbances for even more money. Megacorps, of course, will have their own in-house magical security maintain serious wards around anything really important. Wards above rating 6 should be rare, though, and only for really sensitive locations. Think about that warding ritual: it takes [Force] hours, it's opposed by double the Force, Drain is double the hits on the opposing test, and Drain is Physical if the leader gets more hits than their Magic attribute. Before you set up a high-rating ward, make sure you know who's doing the warding and how much they're willing to risk.