Anyone else think Tuesday Weld was planted by Leo to get close to Frank?
No.
Otherwise, they would've had a dramatic reveal. Frank says, "I want my money!", Leo says, "I own you," and then Tuesday Weld suddenly enters the room and says something like, "Sorry Frank I've been working for Leo all along"

Not much of an actress, but suited this role.
Reminded me of Maria Bello, who responds well to good directors.

Really? It's been a couple of years since I've seen it but I don't recall that ever being implied. I thought that Leo's "family" was unaware of Frank until the diamond deal gone bad towards the beginning?

Yeah but Leo was aware of Frank's illegal activities long before they ever met.
"I can assure you my intentions are strictly honorable."

In their conversation in the diner near the beginning of the movie, he says he's been coming into the diner where she works for 5 months, long before he met Leo.

no that would ruin the point of the film