Recently study some ranges and the recommendation is 3.5x ISO at 15bb with 99.

Villain is a fish. 0% PFR, limp 60% only 10 hands in.

PokerStars - 15/30 NL (2 max) - Holdem - 2 players
Hand converted by PokerTracker 4

SB: 17.4 BB
Hero (BB): 15.93 BB

SB posts SB 0.5 BB, Hero posts BB 1 BB

Pre Flop: (pot: 1.5 BB) Hero has 9 9

SB calls 0.5 BB, Hero raises to 3.5 BB, SB calls 2.5 BB

Flop: (7 BB, 2 players) 4 T 7
Hero bets 5.33 BB, SB raises to 13.9 BB and is all-in, Hero calls 7.1 BB and is all-in

I felt all wrong to me. How would u go in this scenario.

exactly same, not going to check or bet small 99 to give free cards, and fold is impossible

Surely villain has TT or a premium?

I'm not sure why you'd think this when random villain limp/calls pre. I think we see a hand like JT or something much more often than a premium in this spot, but it is what it is.

Math-wise we are good vs Tx and occasional A7, maybe some weird KJ here?

Thing is, that we block J9, 98, 96... I really don't know in this spot. It is interesting and probably villain dependent as well.

I think, that I prefer little smaller sizing on the flop, so that we keep overcards in (maybe that would actually give villain the right price to call... not sure)

Cause the bigger hero bets, the smaller portion of the villain ranges continues (given the probability, that villain's range is at least a lil bit elastic hehe)

Hmmmm, I realized that if we assign to villain's range some 65, A7, K7, this play might be good. I wonder what would GTO say. And what are the players' tendencies in this spot.

Anyways, it is not very common spot, therefore I don't think it matters that much.

Human mind is rather interesting. We tend to solve the hard spots, instead of focusing on "std" spots, where we tend to leak the most hehe. I am not exception obv. It is good to train the mind even in this tougher spots, but it is also very important to balance it out with these "boring" spots.

While this is true, that you should focus on common spots more than some rare spots but this is kinda very common spot for iso pot imo.
Im not talking about mid pocket pair on T7x but having these 2nd pairish hands after iso and how to play them, not a spot you should ignore imo, and figuring out how to play 99 or 7x here is kinda answering a lot questions about this spot in general.