Like let's say someone has 75% of the overall mining power.
That would be an awful lot of hashing power. It would be very difficult and expensive to acquire.
Could they reject other miners' valid blocks?
Yes. As a matter of fact, if they wanted to do so, they could manage it with anything more than 50%.
The 75%'s blockchain will always be longer then anyone else's, so would it cause all other miners' blocks to be orphaned?
If they chose to only build on their own blocks and not accept any blocks from any other miners? Yes.
And they could essentially set rules on the transactions,
They could choose which transactions get confirmed, and which ones don't. They could allow other miners to confirm transactions, and then they could orphan those blocks and confirm other transactions that spend the same inputs instead. They could not create "fake" transactions, or confirm otherwise "invalid" transactions.
and set a minimum Tx fee to get a Tx included in the blockchain.
Yes, they could refuse to confirm any transaction that doesn't pay the fee they demand.
