The feet on the F-style Minimas seem awfully long. How much can be safely removed, before I start undermining the structural integrity?

You're not going to hurt the structural integrity of the guide itself, but the shorter the foot the less contact with the blank and the less wraps to hold the guide to the blank. I can't give you an absolute minimum of how much can be removed before you end up with guides that won't stay put. This is something you'd have to experiment with and see how short you can go before the way you handle your rods results in that foot being too short.

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I also thought they were long when I got my first set. I didn't worry about the length, but the width of the foot was wider than the blank near the top, so I took the time and busted out the dremel tool and skinny-ized all the guides from the top to the bottom.

The feet on the size 4's I have are actually narrower than the 4.0/64th tip on my flyblank. I personally like the added length, and they're still just shorter than my Am Tac Titans in equivalent sizes (5 mm is the smallest Am Tac) and the foot swells in the middle for added wrap failure securtiy. These guides are soooooo much lighter than anything else out there in this style of guide that the added 2 to 4 wraps I use to secure the longer guide foot are well worth it! Incidentally, the opening in my size 4 Minimas "appears" slighly larger that the opening in my size 5 Am Tac Titans.

Torin and Tom are right.

The guides, as they are, will work great. However, the wonderful thing about CUSTOM rod building, is that you can change anything that you want.
I will often shorten a guide foot, or narrow a guide foot if needed to slim it down for a smaller tip section.

The longer the guide foot, generally the easier it is to wrap the guide. However shorten it and or narrow it to your satisfaction. Then, give it a good test - flexing it (on a sample basis) to insure that it is going to give sufficient stability to the guide.

Take care
Roger