There's not much I can think of that will increase the odds of Mercenaries and Smugglers spawning, but here's what I've got:
a) Save scum on the last day at the end of the month, allowing you to reroll what missions the enemy factions launch. I don't recommend this.
b) Take junk missions out of the available pool, such as researching to stop tracking civilian shipping. I'm betting you've already done that.
c) Provoke mercenaries into launching crackdown missions in the following month by shooting down their shipping. I think the mercenaries are guaranteed to launch a crackdown if you shootdown a craft that has 'mercenary contract' as a mission. This doesn't apply to smugglers.
d) If RNG blesses (curses?) you with a mercenary base building mission, let it complete and farm the resulting base and supply ships. Again, doesn't apply to smugglers.
Zappers are a luxury stun gun. Lightweight, depends on the easily trained accuracy stat, can be taken into 0g and infiltration, uses electrical type damage, doesn't require a gal to burn energy to run up to and melee a fool...
It has weaknesses too, of course. Personally I just ninja stick / stun baton / power mace everything because I'm too lazy to diversify my stun weapons.