Would it be necessary to get another rat to house with him, even if he's been housed alone?
I wouldn't add a second rat. I agree with WolfChan, He would most likely reject it.

- How much are vet appointments and such, and do they get any kind of expensive shots? Is a vet necessary? (Need to know because i'm not sure if the pup's vets do small animals too)
To my knoledge they don't need any vaccinations. However it is inportant to have a good rat vet in case of an emergency.

- Would it be okay to switch foods if i'm not happy with what they're feeding him?
Absolutely. Also offer fresh fruit and vegetables (uncooked). Pasta (cooked or uncooked) is also a faverate as a treat.

- What things do you think are necessary to keep in and out of their envornment?

I like WolfChans response to that.

- Is it okay if rats are around dogs? (of course with supervision) I never have, but I would say yes. Your dogs are so well behaved they would fine around a rat. As for the rat I doubt he would mind, they are go getters and usually fear very little.

- How much does bedding/toys/food/everything cost you? (those with lots of rats, what do you think it would cost for 1 or 2?)
I really have no idea. Not too much.

- How long do rats generally live for?
3-4 years

- ANYTHING else you think I should know.
Know that rats are very prone to obesity, so be careful. An excersize wheel is a good idea, however I don't recomend a wire one, they could hurt their feet. The solid plastic ones are much more safe. I also like to use a chinchilla bath house for a bed for the rats, they like it, it stays warm inside (put bedding in it). As for bedding i suggest using Carefresh, it's really safe and softer then wood shavings or pelleted bedding.

I hope you get him. You will have so much fun with him.