First thing, the 400 is designed to be a complete feed, so when you feed that it's not necessary to add anything else. I added the Ultium because it puts weight on fast and if you spread her feedings out to 3 or 4 times per day, you aren't feeding that much at any one time.
Strategy is designed to balance out hay rations, and is designed to be fed with hay. I would not add anything other than the Ultium, which they find to be very palatable, to either ration.
When feeding a couple of hard keepers I found that the simpler I kept it, the better it went for them and ultimately the cheaper it was for me. I have not used Amplify but have a couple friends who have and they weren't real impressed at results vs cost, so I have stuck with what works for me. For my hard keepers the Rice Bran did nothing.
The key, for me, was to use the extra weight formula. Feeding that little bit of extra food (in the 400's case it amounts to 1.25 lbs per DAY not feeding) and tossing in the Ultium at 1 lb per feeding, the weight came on quickly and was not hard to keep on from then on.
So if she was my mare, and assuming I wanted to feed her to the 1400 lb weight limit, I would feed her 15 lbs of 400, plus 3 lbs of Ultium PER DAY. Divided into either 3 feedings of 6 lbs each or 4 feedings of 4.5 lbs each and free choice hay.
To put that in perspective, I use a 3 quart feed scoop which when filled level equals about 5 lbs and a little heaping is 6 lbs. I use a little 1 pint scoop for the Ultium. Each horse then got 1 big scoop of 400 or Strategy and 1 little scoop of Ultium per feeding. This hasn't been an overload on any of them.
Read your feeding instructions on your feed bags, you'll be surprised that you're probably not following the directions on how the feed was designed to be fed. When you start addng this and adding that you can easily upset the balance that is designed into the feed and you find that nothing is working as well as if you went with one thing and stuck with it. And I'm saying that about ALL brands of feed, not just Purina. That's just what you're using and what I've had good luck with.
Use either the 400 or the Strategy, don't mix them as they aren't designed the same. Your worry about too much rich feed is valid if you're mixing, but either one alone or with a small amount of Ultium is not a real rich feed. You are more likely to have a problem by adding sweet feed which is mostly sugar than you are to have one just following the instructions on the bag.
If you're using the Strategy Healthy Edge, they say to feed only 6.25 lbs per day with a MINIMUM of 17 lbs hay per day for a 1400 lb horse. If you feed less hay than that, then you need to increase your feed amount. This can actually end up being pretty cost effective if you are feeding enough roughage. Strategy Healthy Edge is 12.5% Protein and 8% Fat.
With the 400, it is designed to be a complete feed and doesn't figure hay into the amounts fed equation. I think it's easier to figure out and weigh the feed than to weigh hay, especially if it's grass hay, so tend to recommend using this to get the weight on because it's easy to figure. 400 is 12% protein and 5.5% fat.
Ultium is guaranteed 11.7% protein and 12.4% fat.
So, if you add the 11.7 + 12.5 = 24.2/2 = 12.1% protein, and 12.4 + 8= 30.2 = 15.2% fat with Strategy
11.7 + 12 = 23.7/2 = 11.6% protein and 12.4 + 5.5 = 17.9/2 = appx 9% fat with 400.
You still keep the protein down around 12% which is a healthy level and you increase the fat on both which is where the added weight will come from. It's a personal choice which way you decide to go.