A great book to read is Care and Management of the Older Horse, by Heather Scott Parsons. Personally I see no issues with feeding her soaked alfalfa pellets, as long as you are weighing it out, and making sure that she is getting the right amount, factoring in the senior feed.
From what I've been told by several vets, senior feed is actually supposed to be fed as a replacement for hay in the older horse that can no longer chew hay, though it obviously works great in an older horse as an add on to help maintain weight on a harder keeper who can still eat hay. I am not sure how much a "scoop" is of the senior feed, but it is good to be splitting the grain into two meals instead of giving it to her all at once.
I would just keep an eye on her, especially as she adjusts to a new place, and make sure that she eats what you are feeding her, and just pay attention to exactly how much of what she's getting. With my dad's old gelding, we always added the 4-Flex joint supplement to his grain, as well as my Thoroughbreds grain, as more of a preventative measure, then a fix for a problem that was already there. I would highly recommend it even if your horse isn't arthritic, or sore, just to keep things moving right.
As long as she's happy, peppy, and isn't losing a bunch of weight, I would just stick with what you've been doing, and only make adjustments if needed. I think that a lot of people think that owning an older horse means more grains and supplements, and extra special care above and beyond what a horse in his teens might need, but often times the problem lies in the fact that they get too much other stuff when they would be great with less. Get her teeth checked often, as she is older, keep on top of the feet as well. Lots of praise, and love, and as long as she is willing, and able, keep riding her. It will keep her happier and healthier in the long run.
I'm glad that you were willing to take on an older horse, and are wanting to make sure that she stays happy and healthy knowing that she already requires some extra care. Kudos to you. Put up pics if you can.