Anyone know if some Mercs are likely to appear in some maps?

I have been doing merc rush for the past week, doing not but checking Merc gems. I just want to know if there's some mechanic to their appearance because I am losing my goddamn mind trying to find a good KB Clutter and Molten Strike Merc.

I haven't even seen a single Molten Strike Merc in like 50 maps.