Yes, but maybe not like you're thinking...
You've asked us the following (bold added):
I had the idea that I could True Polymorph into a creature that has the Use Magic Device ability, or access to the Paladin spell list, and then cast Find Steed off of a paladin scroll...and then retains the summoned steed afterwards?
So the answer to the second part of this question is an an unambiguous "yes." You would absolutely retain the Steed indefinitely after casting this spell. But since you mentioned "afterwards", I want to clarify and make sure we're on the same page.
Casting Find Steed will take Concentration
Once Find Steed is castfinished casting, the summoned steed will stay with you without requiring you to continue spend Concentration (unlike some other summoning spells). However, the spell Find Steed has a casting time of 10 minutes. And ever since an errata to the Dungeon Master's Guide (DMG), the rules state (DMG, p. 200):
Spell Scroll:...Casting the spell by reading the scroll requires the spell’s normal casting time.
This is significant because True Polymorph is a concentration spell. And the rules for casting spells with "longer casting times" state (PHB, p. 203, bold added):
When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so. (see “Concentration” below).
Since you asked if you could True Polymorph into a creature, I assume that you intend to be the one who casts True Polymorph. If that's the case, then you'll be concentrating on it while transformed. So while you're True Polymorphed, you can't cast Find Steed from a scroll without ending concentration on True Polymorph.
Of course, you could just let True Polymorph's duration finish, and then cast Find Steed afterwards. But once True Polymorph duration is completed, its effects become permanent. What exactly that means could be subject to the views of your DM. The SRD (p. 185) claims that True Polymorph lasts "until dispelled", butdispelled." At the time of the original posting of this answer, that change haswas not made it into the PHB (in spitelisted in any of the numerous Erratas): however, it has been confirmed that the change is present in the newest versions of the PHB (thanks to V2Blast for pointing that out). So if your DM is using the most updated version of the rules, there well be a faster way to change back. However, if they have an older version of the book, there could be an issue. So depending on which sources your DM is using or accepting (or how funny they think this dilemma might be) it might become difficult to change back!
Naturally, this could be circumvented by having someone else cast True Polymorph on you, thus permitting you to cast Find Steed while they concentrate on the transformation. But if you intend to cast both spells yourself, you may find things work a bit differently than you're expecting.
