Yup turn c/f is good here, with fish behind he's almost certainly playing all boats and trips like this on the flop and yeah checking back close to everything you beat on this turn card. The only worse hand he can bet the turn with is going to be 56. He wont be betting this 100% of the time so if you give him like 12 combos of 56 his turn betting range wont be far off from..

equity win tie pots won pots tied
Hand 0: 13.545% 13.55% 00.00% 596 0.00 { 9c9s }
Hand 1: 86.455% 86.45% 00.00% 3804 0.00 { 77, 44, A4s, K4s, Q4s, J4s, T4s, 94s, 84s, 74s, 64s+, 54s, A4o, K4o, Q4o, J4o, T4o, 94o, 84o, 74o, 6c5d, 6c5h, 6c5s, 6d5h, 6d5s, 6h5d, 6h5s, 6s5h, 64o, 54o, 42o+ }

So we canadd a few more combos of like better 7s and 88 and this will still be aneasy fold once we check. Given we only get one street anyways from his weaker hands like 7x in most cases yah your plan is good. Obviously, the more of these hands he bets, the more terrible c/f the turn will be vs betting the turn, but I don't think he has enough worse made hands in his betting range to make this a real worry.

As for pre, 99 is easy strong enough for me to iso here, I'd just 5x it, expect to get the pot HU a good amount of the time and be confident actually betting 99 for value on a ton of boards vs these guys and expect them to play fit or fold post flop, continuing passively with worse pairs postflop, and never not bluffing you frequently. I basically think 99 is a high enough pair vs these to, to play alright even oop 3 ways, I think 88 is closer and 77 and worse is an easy complete.