I've directly copied & pasted your questions & will try to answer what I can for you
Good luck cleaning them up BTW
Q.One has a "fading" symbol; it gets lighter when you warm it and has a hidden symbol underneath. Anyway, her symbol's peeling off a bit on the edges, and it gets especially peel-y wen it's wet. She's got some marks that I want to try and get off, but I don't want to damage her symbol any more than it already is. Should I just leave her be, or go ahead and clean her but be careful of the symbol?
A. If you feel that she does need cleaning, try covering up her symbol with something soft so you don't damage it any more than you have to. Try not to we the whole pony, maybe clean with a cloth, sponge or toothbrush instead to try to keep the water "contained" a little better
If you are happy to put her into your own collection I wouldn't worry too much about cleaning her however.
Q.I see signs of rust on most of their tails. Should I pull their heads off and check inside, even if they're well-attached?
A. As Kaeldre has said, baking soda toothpaste is great for tail rust, if you do pull off their heads to give them a diecent clean, be careful not to sever the head clean off the plug, take your time removing the head & try to gently pry it away. To clean insides soft toothbrush, cotton buds and other long instruments are best as there are so many little nooks & crannies inside a pony to try to clean.
Q.How do you clean the rust on the inside if they have it? What about other gunk?
A. With a toothbrush or cotton buds, baking soda toothpaste is great for rust, I have found bathroom mould killer works great at eliminating gunk, and also helps draw out alot of surface dirt from the plastic, be careful not to get this on your clothes as it will bleach them.
Q. Should I replace the tail washers?
A. If they are really rusted, yes its probably a good idea because they will just get worse over time, zippy ties or similar are a good replacement
Q. A bunch of them have ground-in dirt. What's the best way to get that off?
A. I would invest in a bottle of bathroom mould killer, this stuff is magic on that sort of thing, or magic eraser may work too, both are methods I use alot to clean up ponies that are espacially grungy.
Q. Is there anything I can soak them in to take away the dirt that won't damage their paint or hair?
A. I wouldn't like to comment here, as I would hate to give advice that would damage someone elses pony, so will leave this question for someone else
Q. One has pink highlighter marks. Is there any way to remove or fade them without sunfading? Being the middle of winter, strong sunlight's a bit rare.
A. If you can get hold of a UV or blacklight lamp / bulb, this will do the same thing as sunfading
Q. I suspect a few might have a touch of mold. What's a good way to clean it off?
A. Bathroom mould killer
Q. A few have frizzy hair. Any tips on that? I have a straightener, but I'm a bit afraid to use it on them; don't wanna make the frizz worse.
A. Conditioner, not a cheap home brand one, comb through, if still frizzy there are ways of using straighteners on lower settings, I would however try it out on a really baity one first before trying to straighten a nice pony's hair.