I have a 75w iwasaki merc that was sent by a fellow bulb buddy here. Could I power it with...
A 100w merc ballast with some sort of cap in series with the hot output
Two preheat single lamp npf f40 ballasts in parallel
Two single lamp HPF RS ballasts in parallel?
Any input will be much appreciated, thanks.

I have a 75w iwasaki merc that was sent by a fellow bulb buddy here. Could I power it with...
A 100w merc ballast with some sort of cap in series with the hot output

Better not, such current reduction is rather tricky business (it depend on the exact ballast design and it may be over sensitive to some component parameter)

Two preheat single lamp npf f40 ballasts in parallel
Two single lamp HPF RS ballasts in parallel?
Any input will be much appreciated, thanks.

these will work, but you should make sure the current is OK (some ballast feed the lamps by lower than rated current). Never combine different ballast types, with electronic it does not work at all.

Thanks, Medved. I got the ballasts set up, and it works like a charm, with the ballasts staying cool ish and the lamp at around 85-90% power. The ballasts are two universal 202-B-TC-P single lamp preheat ballasts bought from ebay earlier this year.