On 92 and earlier cars you can use a slow glow relay with fast glow plugs but you can't use a fast glow relay with slow glow plugs.
Assuming the correct glow plugs are used I think your statement is true.
I'm also pretty sure that the Bosch Duraterms are usable with both the slow and fast GP relays.
match the two but even unmatched i think you can start it also,assuming the engine is in good shape
if you use a slow timer with fast plugs, it will shorten their life..
if you use a fast timer with slow plugs, you glow it twice, and off you go..
fast plugs pull more amps. slow relay on fast plugs will likely burn the relay, and as stated before probably shorten the life of the fast plugs.
fast relay on slow plugs no harm done to either.