Couple abilities questions

I'm playing on Old Gen Xbox One. I have a couple questions. I never really messed around with abilities the last time I've had Madden 3 years ago.

First there's a couple players that I've gone into their upgrades and they'll have a fourth or fifth spot and when I click on it there's absolutely nothing there there's no options or anything. Is there a reason for that?

Secondly you don't have to put something in tier one or two or three to be able to use something in tier four correct? Like if someone has a discounted ability and tier four I can just use just that one right? I believe that's the case I just want to double check with the community.

Third what are some good ones to have? I know not to really put anything into your defensive line but I believe the same maybe true for the offensive as well?

Thanks