It seems the rare drops on Gazica are unbalanced as far as Mantis types are concerned. Certainly lower rank mantis (excepting harvesters, inters, menders and sabs) drop few rares (with the occasional scrambler or speed acc).
Higher level stun turrets give good drop on stun charge, likewise marauders for taunt, deto's for charge and droids for droid. Lower level droids hardly drop ( I have yet to see one; but you are pretty guaranteed if someone else is killing them and you are at the higher ranks)
Custodians - very rarely (and not ever in my case)
Resurrectors - as above
Snipers and Signors - occasionally
Rocketeers - the poorest of the poor. Clearly these are unbalanced if you need to take Meth Awakening in order to get gold from rocketeers. There have been drops from higher level rocketeers but very very rarely, and as for the lower ranks it is a complete waste of time.
All in all it seems that the Hive drops more than Gazica. In other systems the more drops per planet could be determined by the amount of activity on that planet (more mantis killed = more chance of drop) but given the amount of activity seen on both Gazica and the Hive there is clearly an imbalance.
I imagine this is a deliberate move by SS to stop players from getting rares too quickly, but given the numbers needed and the cost it is proving to be more of a frustration than a restriction.