Is there a way we can trade stablecoins like Tether and TrueUSD for BTC?
TrueUSD has been added in v0.9.4. See Settings/Preferences/add altcoins
EUR Tether, JPY Tether and USD Tether were proposed in the past, but delisted because never traded.
When the DAO will be operational, you may propose them again (but there will be something to pay for that).
Why can I only add fixed price offers for TUSD??
There is probably no market price provider that Bisq takes for that altcoin. Without it, you can’t set market percentage offers.