H-bridge ic for high voltage and/or current
There seems to be some confusion as to the power requirements. This device does not need to dissipate the energy produced by a 1kV drop at 50A; that power is dissipated by the load.  This device will operates in two states: 1kV blocking at milliamps (or less) of leakage current, and 50A of current at milliohms (or less) of resistance.  That results in reasonable power dissipation levels.  It doesn't have to dissipate 50kW of energy.

At the voltage and current levels you're working at, you should investigate using IGBTs, or Insulated Gate Bipolar Transistors.  A MOSFET has a necessarily small switching region where the electric field (the FE in MOSFET) can work; bipolar transistors have the advantage that the switching region can be a large plane or plate of silicon.  This allows IGBTs to exceed the capabilities of MOSFETs in extremely high-voltage and high-current situations.  This IRF appnote describes some of the decisions to be made between IGBTs and MOSFETs, summarized concisely in this graph:



At 1kV, you're at the upper edge of MOSFET capabilities, and should probably use IGBTs.  You mention that you want to go to 1 MHz "depending on what is feasible" - Most anything can be feasible given enough budget.  I suggest that you try to make an IGBT work for you.

With respect to H-bridge ICs and arrays, sure, there are lots of these.  However, they're spec'ed out to typical use cases.  Digikey claims to have over 3,000 FET arrays.  Unfortunately, the highest Vdss capacity is only 300 V, much less than your 1kV requirement.

IGBTs are more available in arrays too. Digikey shows a few IXYS parts, and their web page does in fact show quite a few parts which meet your spec: Take a look at their website and follow the links to IGBT Modules -> Full Bridge IGBT Modules (their website isn't very good at deep linking).  Here's an example datasheet.  Notice that this isn't a typical surface-mount PCB item; it's a 120 mm by 60 mm chassis-mount module.  These things can put out some serious heat, and can switch some serious power.

You're in exclusive territory here; this isn't something you'll find at your local Radio Shack (not that you'll find much there anyways...).  Finding parts with similar rise and fall times is the least of your worries!