I'll have to consider using shorter leaders then. It seems the 9ft premades are very common, but I suppose I'll have to make my own if I want a specific length.
I've decided to make this build a 3wt, 7.5ft. With that in mind, what length of leader should I make for the small stream situation? Do you have a formula you can recommend? Is there any brand of mono you prefer?
Fwiw the one part of my set up I wasn't worried about is fly size. Brookies are hungry and the little guys will readily hit on a spoon nearly their own size. I had no trouble getting them to bite down on my nymphs. Just couldn't get them to stay on there while stripping in!
IF it was me (and it isn't) buying/building a 7'6" rod, it would be a 5wt which has the desired length, but is a WAY more practical fishing rod for a TON of other places and situations. And if you think a 5wt is too much and will spook brookies on some mountain stream...
Consider that my GO-TO line weight for years on the Letort was a 6wt. Now most often it is a 5wt but NEVER a 4 wt or less.
If you think a 7'6" 5wt will end up MUCH faster and less responsive than a 7'6" 3wt; I have a 7'9" 2wt and a couple of 7'6" 5wt & 4wt rods that you can compare side by side and I think the differences will shock you. It ain't the line weight, it's the taper of the rod.
If you are still apprehensive, make it a 7'6" 4wt for similar reasons.
In regards to leaders...
To ME, a store bought 7.5" leader is still most often too long for really small stream work. In addition, as your tippet gets shorter, unless you carry around a tape measure, you will almost invariably add more than you cut off yielding a much longer overall leader.
When I used to tie most of my own leaders, I took existing formulas and reduced the section lengths proportionally to end up with shorter version of the same taper. Back in the day these ended up around 6 foot and I nicknamed them "Left Bank" leaders because I used them a lot on the steep hill just downstream of the Kiddie Pool at the Little Lehigh.
When I later became enamored with furled leaders, I convinced a friend who was making me standard length thread furled leaders to twist me up some furled leaders only 3 feet long. It may sound funny to think of that now, but back then, NOBODY was making furled leaders that short.
To those 3 foot furled leaders I'll add a short 6-8" transition section and 2-3 feet of tippet yielding a leader of about 5-6 feet which is PERFECT for shorter rods and totally fine for longer rods in tighter quarters.
If you don't like furled leaders or don't tie your own regular leaders, it isn't too hard to cut down store bought leaders a few inches here and there to shorten them. If that is still too much of a hassle,
Harry Murray sells hand knotted 6' indicator (which can be removed) leaders which are excellent.
In regards to rod length and line weight; there are small streams, and there are small streams...
I think many people would consider places like Slate Run or Cedar Ruin small streams, yet you can easily fish a 9 foot rod at either place. However, I know more than a few places where I fish where you couldn't even carry a 7'6" rod around unless it was taken apart, let alone fish it comfortably.
It for THESE places I own shorter rods. Obviously if you don't fish super tight quarters you don't need rods this short, but if you do, you will be MUCH happier going shorter.
That being said, because I like balanced outfits when I can get them, the reels I pair with the shorter sticks inevitably end up being the lightest I can find. That also usually means they are smaller diameter reels that are better suited to lighter line weights.
For that reason and that reason alone, ALL of my rods 7'0" and under are either 3wts or 4wts.