Just to clarify, we are talking about plastic valve caps like these:
So if this is what we're talking about I'm not sure what the problem is. Even if someone has screwed them on really really tight you should be able to get them off quite easily with a pair of pliers. However it might be worth heating them a bit with something like a hair dryer or gas lighter to soften the plastic a bit. I've come across some tight ones in my time but never had a plastic one that wouldn't come off using a pair of pliers.
It might be useful for you to know that the valve assembly is a brass core with rubber molded around it. Here's a picture of what it looks like with the rubber removed:
This is actually an airline fitting I made up to monitor working pressures in the airline to my air tools. I've included a valve with most of the rubber still on it just for reference. You'll notice the metal core goes the whole length of the valve so you'd have to get pretty rough with it to cause any damage.
These caps come in different designs depending on manufacturer and you can also get metal caps - see here:
The metal ones can be more difficult to undo if they've been on for a very long time without being removed and also I've found the "fancy" after market type which come in "pretty" colours can get very stuck. I think this is because they are often made from ally which electrolytically corrodes to the brass valve stem.
So you can probably afford to get pretty rough with plastic ones if you need to. One word of warning though. Some manufacturers use electronic Tyre Pressure Monitoring Sensors (TPMS) which are inside the wheel and are part of the valve - the outside bit you can see looks pretty much the same as a "normal" valve though so it's difficult to tell (some of them are bolted in so you can see the nut but you need to know what you're looking at). Luckily for us it's much cheaper to monitor the tyre pressure by comparing tyre rpms between the wheels using the ABS sensors so valves on these setups are just normal without any electronics and I think all the Fiat stuff does it this way. If you need reassurance about what's used on any particular vehicle I would think most tyre fitting shops or a main dealer would be happy to tell you.
As a parting thought, I've found that many people tend to screw these on pretty tight because they don't want them to come off. I can tell you though, after a lifetime of working with motor vehicles, you only need to do them up "firmly" with fingers only. No need to "super tighten" them using pliers etc. I just "nip" mine up with thumb and forefinger and they never come undone.
Please let us know if you manage to get your caps off and if you decide to try heat be careful you don't damage the paint finish or melt the rubber on the valve stem, It only needs to get hot enough to soften the plastic which will take moments only. Also watch you don't put a naked flame near flamable fluids like fuel etc. I think you'll get these off just by getting a bit more "physical" with the pliers. Maybe try squeezing the cap with the pliers in several different positions before undoing it?
Edit: You'll appreciate that if you decide to use heat then, because I can't be there to supervise you safely, it must be on your own head if you cause damage to the vehicle, yourself or others.
