Random Battles

An idea I've been playing around with in my mind and thinking about implementing;

I think by and large that boss encounters and other special events are what makes Final Fantasy games really shine and lots of rom hacks prove this. Incidentally Return of the Dark Sorcerer does this incredibly well, I find, but it got me thinking about the trash mobs in random encounters, their main purpose and how I can alter them in general in interesting ways to keep myself and potential others who might play my rom hack interested in the random encounters as well.
Mainly, they're used to drain your characters or HP, MP, resources and sometimes throw something nice your way as well as give you a sense of progression as you level up and defeat them easier only to be slapped down by the next big-bad to put you in your place. But I was wondering how well people would respond to, if, they served a purpose of having unique or unusual weaknesses that could be exploited as a subtle hint as to what the upcoming Boss might be vulnerable to as well. For example; A dungeon has more single enemy encounters but they're all vulnerable to the Imp spell with the weakest group being Imps or impish. Then the boss fight has support minions or is himself susceptible to Imp to even the playing field whereas without would prove far more daunting whereas I think most people accept that Bosses are immune to most things (except when they aren't) and generally overlook even trying.
The question here would be; Does that seem like something people would pay attention to or would it just seem frustrating and random? Or on the other hand be too obvious and just adds more work load with no extra pay off?

Then there's just a few enemy types I think would be interesting;
Very fast enemies with very low damage that get frequent attacks.
Example: they hit you 5 times and do 50 damage each and it's annoying seeing the enemy get so many turns you want to take them out first.
But then sometimes it's paired with a slow guy who does twice the damage in the same length of time with one attack.
Or like 6 of them that do like 5~15 damage but combined pull out 1000 damage throughout the fight. They aren't very threatening to your parties survival but definitely drain your resources undoing that damage.

How has everyone else handled modifying the random encounter enemies?