If a non-NCO is a team leader, do you have to stand at Parade Rest when speaking to them?

Do you have a problem standing at parade rest for a TL? If so, you're going to have a very difficult time.
What is the standard in your unit? If it's standing at parade rest, do it and be part of the team, or push back and accept the label of barracks lawyer and trouble maker.
If you're asking if it is in writing in an AR or FM, the FM (D&C) states NCO, not a person assigned to an NCO position. AR 600-20 places NCOs and enlisted member in leadership and supervisory positions in the NCO Support Channel, but doesn't address your question.
Still doesn't say you "have to stand at parade rest," right?
If your Chain of Command wants you to stand at parade rest for the SPC in a TL position, and you don't, it may be a stretch, but it could be considered conduct that interferes with the good order and discipline of the unit. That, is in writing under the UCMJ. Yet, I'm not a member of the JAG Corps, or a barracks lawyer.
To put it in perspective, I recall standing at parade rest for NCOs who were junior in rank to me in Airborne School as a SSG for SGT Blackhats, and as a SFC in ANCOC, when BNCOC SGLs engaged me. Sure, a couple of the BNCOC SGLs did it just to have a SFC stand at parade rest, but you have to be above that.
Don't spend time wandering whether someone can legally do something, aside from having, discrimination, or safety violations. Spend time on making a positive contribution. You'll find it pays off much better than pushing back against things that will matter little in hindsight.

Yes, if you know they are serving in an NCO capacity, then they rate the courtesy's afforded an NCO. Refer to AR 600-25 Salutes , Honors and Courtesy's and AR 600-20 Command policy under Duties of NCOs.
I served as a Squad Leader as a SPC4 in the 80's and since Combat Heavy Construction units were not authorized Corporals was appointed as an acting Sergeant. Wore SGT Stripes and carried appointment orders but was paid as a SPC4 ( there are no longer acting Sergeants). That removed the issues that have been discussed in a couple recent threads where SPC's and in one case a PFC were serving in team leader positions.

Required, per normal customs and courtesies? No. However, you should tread cautiously here. Are you searching for a technicality to avoid showing someone in a leadership position some base level of respect, or is this someone in a leadership position who is trying to lock you up to prove a some kind of a point? If the former, I would recommend against falling on your sword over this. If the latter, I would comply when this next occurs, but then bring it up to your NCOs afterwards.