MOSFETs in high voltage low current applications
is a high voltage MOSFET appropriate for this application, keeping the
requirement of low leakage current and fast switching in mind? Is
there a better component? (I have purchased a Vishay IRFPG50 (1000v,
6.1A))

The types of components that are appropriate depend on what you need your switching to look like.  If you just need to turn on quickly and drain the whole capacitor into the load then you could use either an IGBT, MOSFET, or a mercury wetted relay.
If you also need to turn off quickly to control the pulse width or deliver multiple pluses then only the MOSFET is suitable.  IGBTs often have long turn off times (a few us), whereas MOSFET's are capable of both fast turn on and turn off (ns to 10s of ns range).
As far as leakage goes, a mercury-wetted relay is going to be the lowest leakage in the off state.  MOSFETs can also be as low as a few uA worst case.  Silicon carbide MOSFETs tend to have lower leakage than regular Silicon MOSETs.  For IGBTs expect leakage in the range of a few 10s of uA.

To limit the leakage current, where and of what value resistor should
I add to the circuit? Reading other posts, I have heard discussion of
adding a resistor to the gate of the MOSFET. What would this achieve
and are there other similar common practices I should know about?

For MOSFETs the leakage is usually specified at maximum operating voltage and with VGS=0V.  To decrease leakage you can get a device with a higher operating voltage relative to your working voltage, and the leakage will decrease accordingly.  Also, driving VGS to a negative voltage (say -10V) will decrease leakage even further compared to just making VGS=0V.
The IRFPG50 probably isn't appropriate for your application.

Its only rated for 1000V.  That means you have no safety margin if you operate at 1000V.
Its only rated for 24A peak current.  For any loads less than 40 ohms you will exceed the rated maximum.
The datasheet says 100uA of leakage current at VDS=1000V.  You didn't really specify how much leakage was too much, but 100uA is pretty high.
Some distributors are listing the IRFPG50 as obsolete.  So if you plan to build more of these you may have trouble finding them in the future.

Some alternatives might be...
C2M0045170D , 1700V, 45mOhm, 160A peak, 2uA typical leakage
https://www.wolfspeed.com/downloads/dl/file/id/960/product/207/c2m0045170d.pdf

https://www.digikey.com/en/products/detail/cree-wolfspeed/C2M0045170D/6148740
STW21N150K5, 1500V, 0.7 ohm, 56A peak, 1uA max leakage
https://www.st.com/content/ccc/resource/technical/document/datasheet/0f/96/82/a0/4d/08/4b/18/DM00130400.pdf/files/DM00130400.pdf/jcr:content/translations/en.DM00130400.pdf

https://www.digikey.com/en/products/detail/stmicroelectronics/STW21N150K5/5455741
DRIVING:
For driving the gate, you can either use an isolated gate driver IC, or a pulse transformer.
Some possible gate driver ICs include...

Analog Devices ADuMxxx series.
ADuM4122, 1660V peak, 2A drive, 150kV/us immunity, 48ns tpd, magnetic
https://www.analog.com/media/en/technical-documentation/data-sheets/ADuM4122.pdf
Infineon 1EDxxx EiceDriver series.
1EDC60H12AHXUMA1, 1200V, 6A drive, 100kV/us immunity, 300ns tpd, capacitive.
https://www.infineon.com/dgdl/Infineon-1EDCxxI12AH-DS-v02_00-EN.pdf?fileId=5546d4625d5945ed015d88ad8a38235e
Silicon Labs Si826xxx series
SI8261BCD-C-ISR, 1414V working voltage, 4A drive, 35Kv/us immunity, 60ns tpd, capacitive
https://media.digikey.com/pdf/Data%20Sheets/Silicon%20Laboratories%20PDFs/Si826x_Rev2015.pdf
Texas Instruments
UCC23513BDWY, 1500V DC, 4.5A/5.3A drive, 150kV/us immunity, 105ns tpd, optical.
https://www.ti.com/lit/ds/symlink/ucc23513.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe&amp;ts=1606534057238
UCC5390ECQDWVRQ1, 2121V DC, 10A drive, 100kV/us immunity, 100ns tpd, capacitive.
https://www.ti.com/lit/ds/symlink/ucc5390-q1.pdf?HQS=TI-null-null-digikeymode-df-pf-null-wwe&amp;ts=1606533698123
ISOLATED DRIVER SUPPLY:
All of the above drivers require a floating isolated supply to work.  Analog devices does offer one IC with a built in isolated supply, but its not rated for 1000V so you must use an external one.  Some possible options are...
MGJ6D241505WMC-R7, 3kV working isolation, +15V/-5V, 15pF isolation capacitance.
https://www.murata.com/products/productdata/8807030063134/kdc-mgj6-14mm.pdf?1583754812000
MGJ2D121505SC, 2.4KV working isolation, +15V/-5V, 3pF isolation capacitance.
https://www.murata.com/products/productdata/8807029997598/kdc-mgj2.pdf?1583754812000