Here are some answers and a web site to go to to help you with a Russian tortoise.
"is it true that water soaked dry dog food is a good tortise treat?" Only for forest dwelling tortoises like Redfoot's Yellowfoots, and Hingeback's. Russian's are NOT a forest dwelling tortoise. Feed it to them and that means $$$$$ at the vet for you.
"is fruit bad for tortises? can i give it to them for a treat? Once a week?" Only for forest dwelling tortoises like Redfoot's Yellowfoots, and Hingeback's. If you give fruit to arid dwelling species, like a Russian, you're asking for trouble. The ONLY exception is prickly pear cactus fruit. Fruit causes parasite bloom in their guts and that means $$$$$ at the vet for you.
"Is a 75gal "fish tank" okay with proper ventilation and such?" No. Tortoises try to go where they can see so you'll have a tortoise trying to get through the glass all the time and this will stress them tremendously and that means $$$$$ at the vet for you. Depending on the size of your tortoise, either a 50 gallon Rubbermaid bucket (which cost much less than a glass tank, and you can get at Target or Walmart) or a reasonably sized wood enclosure is much better for tortoises.
"what is considered 'winter'?" Depends on where you live and the species of tortoise you have in your care. For tropical species it would be a 65-70 degree tempurature during the day and 12 hours of sunlight. For Mediterranean species it would be daytime temps below 60, which will cause them to want to hibernate.
"what is better, a indoor "fish tank" 75 gal) or pen?" See my answer to your third question.
"i have read information on in the russian tortise, and it kinda scraes me on the shell rot. is it really that common, and is it easy to get rid of??? " If you have them in a wood enclosure or Rubbermaid on a 4-6 inch depth of a 50/50 topsoil/play sand mix as the substrate, a tempurate range from 70-75 at one end and 95 (under a Mercury Vapor UVB/Heat lamp - PowerSun of ZooMed UV heat are best), with a hide box, you won't have to wory about shell rot. If you keep them on a damp or wet substrate your tortoise WILL get shell rot. It's easier on you and your tortoise by not getting it in the first place.
Keep Russian's in a dry, warm environment where they can dig into the substrate and/or hide in a box and they will do just fine. Just don't feed them fruit or dog food. Go to www.russiantortoise.net to find out how to feed them correctly and learn all about Russian's.
Good luck!
PS. If you get one, take it to a vet who knows something about reptiles right away. Your new tortoise will be need to be checked for various parasites and worms.