It's a plural. Also I think it's in the construct state, but that would relate to the first word, not the second, unless my grammar has escaped me completely. It means something like "salad of tomatoes".
Saying in general that word order doesn't matter isn't correct. There are situations where it doesn't matter, but there are also others, where it does matter. You simply can't say עגבניות סלט, just like you can't say "salad tomato" in English.