Can I use an SCR such as BT151 or BT136 (or even a triac such as TYN616), to pass a high voltage(400V) high frequency signal (pulsed DC at 50KHz) for a fixed period? such as 1 second on, then off for another second, then on again?
No. The off time is not enough for the thyristor to turn off AND the rate of rise of voltage (dv/dt) is going to be high enough to turn the thyristor on even without gate current.
Both factors are too high for general-purpose thyristors by at least an order of magnitude.
Use a MOSFET, a BJT or an IGBT. Or even a relay.