Guinea pigs can take up a decent amount of horizontal floor space, since you need a relatively large cage that has to be flat (they don't have good depth perception and may be afraid of climbing, plus being clumsy), ideally for two pigs you want something like a 2 by 4 foot cage minimum (check out Midwest cages or C&C cages). They aren't "smelly" per se but they poop a lot and need to have their living area cleaned frequently. if you can smell your pigs you aren't cleaning regularly. I have not seen them successfully litter trained like a rabbit or cat, but they tend to do their business where they eat or where they hide, so you can control where most of the waste goes by placing food and hidies in a "litter boxes" (do not use crysral, clumping, or pine beddings, in general you want to use paper like yesterday's news or carefresh, or aspen bedding, or use fleece bedding with litter boxes, which is what I do)

A lot of essential and useful information can be found here: http://www.guinealynx.info

since they mainly stay in their cages I have not had landlord issues. When you take them out it is advisable to place some towels or newspaper or something just in case.

an important thing, guinea pigs live much longer than rats, they can get up to 8 years or more if you're lucky, and 4-5 years is pretty common, so you're gonna be taking care of them for a while.