I know about 1% about X-ray machines so take this advice as a bump.
You have a 100 amp oanel already loaded to 50 amps and you want to add a 100 amp load to it? 50 + 100 = ?
As for the computers, the standard advice is to use an UPS system. They should continue to work for at least fifteen minutes after your main breaker blows.
As for the 250 amp start up, this is no problem to me. #10 wire can take 10,000 amps for 1 second. [Don said so.]