Mains Powered Ionisation Smoke Alarm with Alkaline Back-up Battery - BRK 770MBX

It is necessary that an electrician installs mains powered smoke alarms.

Yes, by pressing the test button on one alarm all the units in that family will also be activated. To 'teach' the units which other smoke alarms belong to the same family, the smoke alarms have to be set up together during the installation. This is done by pressing a 'housecoding button' or similar process.

No, to stop the alarms from sounding the hush button on the unit that started the process needs to be pushed. Pressing any other unit will just silence that one unit. The unit can be identified by the rapidly flashing red LED.

The sticky pad is suitable for all battery operated smoke alarms. However, we cannot judge if battery alarms are suitable for your let property, as let properties usually require mains powered alarms. There is some advice to this in the governments guides to fire risk assessments. http://www.safelincs.co.uk/Fire-Risk-Assessment/
Also see the LACoRS guide for let properties.

The 9V alkaline batteries and AA alkaline batteries generally last 12 to 18 months. Lithium batteries will usually last 5x longer than their alkaline counterparts, achieving around 5 years lifetime. Some lithium cells are also rechargeable, and when used in a mains-powered alarm may last for the entire functional lifespan of the alarm. Newer alarm models may also feature a "sealed" lithium battery. These batteries are specialist components which are designed to not be removed or need replacing by the user, and have been tested by the alarm manufacturer to ensure they last for the lifetime of the alarm.

Please note: the actual lifetime of your batteries depends on how often the alarms are tested or activated, as sounding the alarm draws more power than when the alarms are "waiting" to detect fire. You should also read your alarm's manual to make sure you only use the recommended types and brands of batteries, as failing to follow these instructions may cause the alarm to malfunction, not alerting you in the event of a fire, and might also invalidate the warranty.