I doubt any professional installer has used or can give much advise on cheap kits like that.
Beams I have used have adjustment for this sort of thing but I would expect them to detect any break in the beam especially at walking pace otherwise not FFP IMO.
I would be putting both in LC and SCB modes to reduce load on the panel.
Without checking with a multimeter you have no way of knowing the system load and if the standby battery is adequate.
You need to use 360 PIR's that are designed to connect to an intruder alarm system.
Many manufacturers make 360's, from memory the most discrete being Scantronic 450, if they still make them.
There are companies listed here you can try.
http://www.thesecurityinstaller.co.uk/community/forum/268-uk-security-installers-by-regional-police-force/