Easiest Alien police to make would be from slizer the twins and snake's species. hardest to make would be one with frenzies, but nobody would want to do that seeing as how people think his set stinks.
As for the helmets, here is my theory. the moon has a slight atmosphere (due to teramorphing) and beings who need air can survive without a helmet for a while, but with a helmet, indefinately! and some of the aliens breathe air but the humans do not make helmets that fit their heads so must live in atleast partialy enclosed arias to replenish their air. other aliens (slizer, and snake, maybe the twins) breath different gasses, thus they need helmets.
the aliens aer not alowed to live in the cities, because the humans are slightly zenophobic, or maybe they are but aren't treated very nicely, so many don't. either way the alient have resoted to crime to get what they want.
Unlike many people I LOVE the mini ships, I always have, and I am sure to get squidys pitstop, or even 2 (not likely) miniships have always been my favorate, though i'm not to keen on the scooters, the ship that squidy rides, might just be my favorate in the line, followed by the turret sized spacecycle in max security transport (no lie) but i'm not sure i'll get the latter. maybe just because it has slizer. snake's turret like scooter isn't awfull though, but snake is far to common! i'm tempted to use my extras as unnamed mooks and bouncers, and generic aliens at the modded squidies chop shop. I do like the ground vehicle used by twin one. that's perfect to fight the police rover in galactic enforcer (or whatever) sorry for the long post!