So the question is, Is there any harm done using combination of 18W T8 and 20W T12 in 230v 40W choke ballast or will it just work out fine?

If I remember correctly, I believe I did see such a combination and it seemed to work. I don't recall if I saw it in person in Europe or on the Internet, though. I can't see why it wouldn't work.

Primarily I was wondering does it have any significant negative effect on lamp life. Ive been running such combination for some days but rarely left such combination to permanent.

I remember someone asked somewhere here this same question years ago but I dont remember what was the answer and did not find it anywhere.

I can't imagine it would have a significant impact on lamp life, but I really don't know. One lamp would likely be slightly overdriven and the other slightly under driven, but I don't suspect it would be enough to have a big impact. Maybe in a large installation where the lamps are used a lot, you would notice that one type burns out faster than the other.

It won't be a problem in continuous on mode. Whatever variation in working current would be, it is way below the tolerance of the lamps, and below the difference between running 2 lamps on 36/40 choke vs 1x 18/20

What i would check however is, how blink happy the startup is. That would have effect where lamps are switched on and off more often



"Maybe in a large installation where the lamps are used a lot, you would notice that one type burns out faster than the other."
I think that would result from differences in the quality between the T8 and T12 lamps (as i can imagine, different models & manufacturers), more than anything related to the series connection

The T12 will behave like T12's use to do on a series circuit, the T8 will do what T8's use to do.
In other words: Don't worry, no problem related to this combination at all...