100NL: TT on K high board vs donk

I'm a FR player making my third attempt at 6max.
I'm very heathly rolled for 200NL (about 55 buy-ins) but I want to try and get the grips at 100NL before.

$0.5/$1 No Limit Holdem
5 players
Converted at weaktight.com

Stacks:
UTG ($240.70)
CO ($137.10)
Hero ($100.00)
SB ($107.10)
BB ($47.50)

Pre-flop: ($1.50, 5 players) Hero is BTN :tc: :td:
UTG raises to $4, 1 fold, Hero raises to $13, 2 folds, UTG calls $9

Flop: :kc: :7c: :3h: ($27.50, 2 players)
UTG bets $1.50, Hero calls $1.50

Turn: :7h: ($30.50, 2 players)
UTG bets $20, Hero calls $20

River: :7d: ($70.50, 2 players)
UTG bets $45, Hero ???

Q1: My image is TAG.
villain was running 54/28/2 (over 100 hands).
Villain stacked me 30 mins ago when I squeezed OOP, he called with 45o and hit pair + OESD on the flop and he pushed my c-bet and I called. So he makes pretty loose calls Pre.
However, I have not seen villain show down bad hands yet.
His min bet on the flop looks a lot like the nuts or FD. But when he 3barrils, I believe we have to let it go, right?
I know you guys will say, raise that flop bet! But I felt that if villain called and checked the turn I would have to check behind on the turn to get some value on the river, but maybe it would be better to gain control of the hand?
Thougths?

Q2: I'm playing at Ongame and there they have only 5max. Any thoughts of particular difficulties of playing 5max instead of 6max?

Thanks!

My thoughts are that if you raise the flop bet a good chunk, he calls and checks on the turn he's on a draw. With your 3 bet you could easily be on KK or better and got your house. If he reraises you it's likely he got AK or KK and you let go. If I raise ppl with PP and come up against significant resistance with overcards I tend to believe I'm beat and fold rather than slug it the whole way, though that could be a little too much on the cautious side. Then again, this guy is pretty loose and if you have seen him raise up with small pairs before you could be good. Your call on flop could have given him the impression his 66 is solid.