If you feed a pre-ground flax, it needs to be stabilized otherwise it will quickly go rancid. We have a great locally produced flax so I haven’t used any of the more popular products like Omega Horseshine etc. but so long as it is stabilized, you should be good to go.
It is worth it to do the math and figure out how much Se your horse is getting if you’re feeding multiple products with added Se. Toxicity is no joke, so best to figure out the actual amounts.
As far as adding more E, supplementation has been shown to be quite safe at very high levels - up to 10,000 IU / 500kg of bodyweight daily - so you’re likely very safe to up the E, if you like.
I personally feed 3000 IU of D-Alpha Tocopherol (natural Vit E as opposed to synthetic DL-Alpha Tocopherol, as it is more readily absorbed). E really needs to be fed along with a fat for good absorption, so you are on the right track feeding it along with flax. The other option is feeding one of the water soluble E supplements like Elevate or Emcelle, but they are quite pricey.