I'm pretty sure all 51+ mobs in the planes summoned. Damages had to be done for it to occur. I believe the mob had to lose 1% of its hp for it to start summoning. I do not recall getting summoned on throwing daggers/bow shots. I remember soloing lvl 48-49 mobs in PoF on my druid and never getting summoned.
As far as rats goes, they should be between lvl 48-52, like most plane mobs
Not all 50+ mobs summoned. I'm sure some research will provide you with more information
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