Do you need low level minions? Yes
Having construction workers let's you build rooms, place objects, etc. Also having a large base of construction workers allows you to have train more people into the other tiers faster than if you had only a few.
Valets are important for putting out fires in your base and technicians repair objects in your base to prevent explosions. Higher tiered versions of this minions won't do these jobs.
My general ratio was about 20 construction workers with about 26 in each type of minion class (military, social and science).
For Military I go 5 -> 5 -> 8 / 8
For Social I go 9 -> 5 -> 7 / 6
For Science I go 9 -> 5 -> 7 / 6
I kept more valets and technicians around for the special tasks listed above.
Adjust the roster as you see fit. If your base is being attacked more, go with more military. If you want to research more, get more science minions.
Keep in mind, higher tiered minions can teach lower level minions, like a diplomat training a construction worker to be a valet, so keeping your minion roster higher tiered heavy is a good idea.
As a side note, there is a fan made mod called Evil Planet fan mod. Link here That increases your minion capacity to 150 amongst other things. You might want to check it out.
Source: Evil Planet Wiki