Before you got much further, I want you to consider the possibility that you already know the answer. This is possible because you already told us the answer:
If they took the ability to cross over into the NeverNever (the spirit world) then that would tell me they wanted NeverNever shenanigans, if they took Spider Walk then that would tell me they wanted scenes with opportunities for unconventional acrobatics...
When players create characters, they are telling you what they want to do in the game. Your players might just be saying, "We want to solve our problems by tearing them apart with our bare hands!" That's OK! You can still tell interesting stories with this being what your players want!
I assume you will take the advice of others and ask your players, "Hey, is this really what you want? Or do you think that if you don't do this I will TPK you? Because that's not really what FATE, DFRPG, or my game is about."
Assuming you've done that and yes, they thought they were being pretty clear that they wanted to build physically powerful protagonists for themselves, there are a lot of tools available to you as a GM within the game for getting good stories told.
First of all, make sure that when building characters, the players make interesting, compelling Aspects. That's where your story-fuel comes from, not really their powers anyhow. Are you sure you're using the guidelines that make sure some of those Aspects cut both ways? Are you tying them to each other, the NPCs, and the setting?
Next - some of those building block powers have a Catch - A circumstance that limits the utility of those powers - but Strength does not. Lucky for you, it has it's own, implicit Catch: Amazing feats of strength attract Unwanted Attention. I capitalized "Unwanted Attention" on purpose. Mortal authorities are going to notice beings that hurl cars around and knock down walls with their bare hands. And the attention of mortal authorities is Bad News in the Dresdenverse. This Unwanted Attention can motivate them to use other solutions, and can actually lead to some interesting problems.
Also - characters that can dish out massive physical harm are expected to do so in the service and defense of those who can't. That's what makes them heroes. All of that Unwanted Attention therefore means that those who need their help will tend to seek them out. And no matter what their goals may be, if they're good guys, like Harry, they'll just have to find some way to help the helpless before (or maybe during) pursuing their own agendas. If they won't they're monsters, not heroes.
Finally, you said this:
...every character seems to use up the "spare change" of refresh on these powers.
Spare change? What? Refresh is a superpower. This is your third game, right?So your players understand that Fate Points are narrative power. Low refresh means, "Hey, Mr. GM, I'm going to get kicked around for a while at the start of the game. I'm going to make some bad rolls and just have to suck it up. I'm going to take some Compels and maybe even seek out some Compels, so I can get a pool of Fate Points built up. Use this time to get some drama accomplished - and then I can go kick ass!"
I'm not saying the players aren't operating under false assumptions. I'm not saying you might not have some stuff to work out out-of-game. I'm just saying that Trolls and White Court vampires and other super-strong good guys can totally work within the framework of the game and might just be what makes your players happy.
Stop Co-GMing.
Clearly, there are at least two (and, it sounds like there may be three) competing ideas for what The Campaign should be. Since you've tried (and failed) to convince Tim that his "tweaks" and loot are making it impossible for you to run your sessions in the same campaign, stop sharing the campaign: his style and yours are simply incompatible (note: neither is necessarily "wrong" or "bad", just "different").
Depending on the desires of the rest of the group, there are two major options:
- you can bow out gracefully ("I'm sorry, this isn't for me, but have fun!")
- your group can rotate campaign worlds (and, perhaps, systems) instead of just GMs
- on a regular cycle (weekly/monthly/...)
- between "chapters"
- between "campaigns"
I'm in a group that's rotated successfully for years, even with slightly different players (one person worked a rotating shift, so was only available "2 on, 2 off"; that helps); I've also had friends gracefully bow out without causing problems re-joining later when a new game/campaign was starting. I've also had friends who had a long-running "God-mode" campaign interspersed with "Krazy Kobold Adventureses", which acted as a palette cleanser between "chapters" (in that particular case, they were explicitly in the same world, but the Kobolds heard about fantastical things that were happening "way over there" and never directly affected them).
Best Answer
If your GM doesn't mind, there's no reason why a centaur couldn't work. You'd want to keep him roughly balanced with other races; 4e doesn't really have the same concept of monster levels as 3e did, so he needs to be on par with other races from level 1 onwards.
Building Your Own Based On Another Race
Each 4e race gets +2 to two stats. For the centaur, I'd make it Dexterity and Strength -- they're strong and agile, if I recall correctly. Constitution would be a decent alternative to Dexterity, if you want to emphasize endurance.
Next, I'd take a look at another race that has similar characteristics. In this case, how about the minotaur from Player's Handbook 3? The minotaur has a charge power, which seems to make sense for centaurs. They get skill bonuses to Nature and Perception. The former is a good match for a wild race like centaurs; Perception might not be as good, but you can change that to any other skill without causing any problems at all.
If I were doing this, I'd also want the centaur to be faster than most races. Minotaurs are speed 6, which is exactly average. If you make the centaur speed 7, you should drop some other class feature. Minotaurs have an ability called Ferocity to reflect their bloodthirst: when they drop to 0 hit points, they get to make a free attack. That doesn't seem to match the centaur all that well, so maybe drop Ferocity and take the extra square of speed instead?
The other advantage of using a minotaur is that you could keep many of the minotaur feats. For example, there's a minotaur feat called Springing Charge that allows you to charge again when you score a critical hit with a charge attack. Normally it's minotaur-only, but there's no reason not to let a centaur who's based on the minotaur use it.
In some cases, of course, the minotaur feats won't make sense. Ferocious Frenzy is a feat that builds on the minotaur's Ferocity power, and if you don't give that one to the centaur, the feat doesn't make much sense either.
The basic concept of taking something that's already been written and renaming it to be the idea you want is called reflavoring in 4e terms, and it's immensely useful for all kinds of things.
Building Your Own From Scratch
Now, if you want to put a bit more effort (and some money) into it, there's an absolutely great PDF called Hard Boiled Cultures, which breaks down ways to balance new races. It's pretty cheap and I recommend it to anyone who wants to read some smart stuff on adjusting cultures in 4e, but it's definitely not the easiest way to get a playable centaur.
Third Party/Fan Material
I can't vouch for the quality of any of these, but:
Man, definitely a tendency for fan-built centaurs to have the fey origin. If you like that, you could add it to the minotaur reflavoring above with no issues.