Nicole.
I cant do the whole multiple developer thing. Financially, and because of the confusion it causes me.
What subjects to you normally shoot?
in 8x10
I shoot alot of old buildings, abandoned places. Im as in love with overcast dreary days as I am with beautiful blue sky days.
in 35mm, medium format
lots of concerts in really low light using fast cheap lenses. sometimes I like to walk around the city and just take photos of anything and everything.
What film do you use and why? Do you generally stick to the same film or vary according to subject/conditions?
8x10
Ultrafine 125. Basically Im used to the film and have been very pleased with the quality and also LOVE the price. I stick to this film in 8x10 almost exclusively. Im used to the developing times and am becoming more and more accustomed to the results and the correct way to expose it.
35mm and MF
Ilford 3200, TriX, HP5, PanF. depending on the lighting conditions. If I know the light conditions will be diverse Ill use the TriX or HP5 because of how forgiving they are. I recently illustrated an 8 page article on a soldier that was badly wounded in Iraq and recovering here in Central Texas. I shot the interview with my canon 35mm, a 50mm f1.8 lens and some triX I shot the whole thing in manual mode and the exposures were all over the place. Yet I developed the roll at the median of about 800iso and got resulting gorgeous negatives that fit the copy and the piece wonderfully. Even some of them are full two page images and the images are sharp with noticeable grain... but that was what was desired.
The Ilford 3200 is wonderful for low light concerts and the like. Ive successfully pushed it +2stops and enjoyed the results. Also liked the results of it being shot at 1600 and developed normally.
I also love PanF for landscapes/architectural work.
The TriX and HP5 is wonderful for day to day shooting and city stuff.
What developing agents do you use and why? Do you generally stick to the same developing agents or vary accordingly to film/subject?
I use Pyrocat HD 1:1:100 for 8x10 and 4x5. I brush develope with development by inspection method. Minimizes scratches and creates the most even development.
I use rodinal with all my medium format and 35mm film. almost exclusively at 1:50 or 1:25 if Im in a rush

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Does various types of developing affect the archivability of the negatives? If so, how?
Not that I know of.
I hope that helps. Im by no means an expert and likely theres alot of more tech savy gents/ladies on here that can give more info.