you should have enough space. graco car seats come with booklets that tell you which cars and which seats you can use them in... if they are tiny ones in rear facing carseats ones with bases are ideal and can be used with either lap or diagonal and lap belt config. britax do a forward facing seat that can be used on just a lap belt too.
other option is halfords or mothercare both do safety check things to make sure carseats are fitted properly......
my older 2 both had booster seats when we had littlest in rear facing seat, so not as big a problem.
as long as seats in boot are forward facing you should be ok too. it's the sideways dixie seats that are a prob as the car seats aren't tested on them, so they can't say whether it's safe or not.
hope all this makes sense and helps...
it's probably as clear as mud tho, as have been up for hours and have poorly child moaning in my ear!lol