Intro to soda questions.

So, for Christmas my parents were nice enough to get me a two keg kegerator conversion kit and I was hoping I could start doing soda now with force Carb. So, my questions are whether or not having soda in one keg will make my co2 tank last a lot fewer kegs. I only have a 20oz tank so I'd like to be able to get both kegs out of one fill.
2) I know people say that rootbeer will get flavor stuck in the lines and the kegs, so does this hold true for other sodas? Like, will I get rootbeer taste in my cream soda as well? Or if I do Ginger ale or cream soda will it get flavor into my beer as well?
3) will a 20ft or 30ft hose be better for a soda only keg?
Thank you guys for your great advice.
-Connor

I run my soda at 37 psi, that's 3-4 times as much CO2 as for beer.

Soda flavors do get stuck in the plastic. Just reserve one tap for soda and it won't be a big deal.

I use 20 ft of 3/16ths.

So, even though the soda flavors get stuck in the plastic, would I notice it for going between different sodas, say rootbeer to cream soda to ginger ale? And thank you for the information. I believe I'm going to try and upgrade to the 5 lb tank.
-Connor

I run my second keg as carbonated water and add the syrups when water is dispensed. This would also stop the issue of the keg having a soda flavour stuck in the lines.

Not sure about the line length I'm afraid. Compromise and go with 25ft?