Being mobbed no longer gives more defence skills its actually just causing you decay for no gain
Having many mobs on you is pointless unless your using a splash dmg weopon which will basically keep all mobs in a state where they can give you skills. Once you've received a few green line skills from a mob it stops giving them unless your engaging with it ie shooting it.
I dont know if extensive testing per mob has been done and made public, it probably has, but evade tt value is likely related to speed of attack and mob level/dmg. Those punies as an example although giving you skill, the tt value of the skill would probably be less compared to a single green line of skill obtained from a higher level mob. From now on try and ignore the green lines and concentrate on the tt value of your skills, using jdegrees chipping optimizer in connection with a skill scanner is the way to go.
For your comparison, below are two recent examples of how different mobs can affect evader skill, in the below it is measured in ammo shot as that is how i track, it doesnt tell you the tt value of the evader skills each mob gives, but how much evader skill tt value I actually got from hunting that mob over a period of ammo being shot into them. It takes a lot less time to kill an allophyl than a trox, so the allo spends less time hitting me, it has slower hit rate and is a lower lvl mob, i think all this probably affects the tt value of the skill a mob gives, i dont know this as fact though. But to answer your op question, i think this may help.
.27tt ped evade skill per 5k ammo on small allophyl
.79tt ped evade skill per 5k ammo on medium atrox