So why do you say מרק עגבניה but סלט עגבניות? Is it just how you say it in hebrew or is there some rule behind it?

Tomato salad! Never heard of that before. :)

Is this sentence okay?: הילדה ואני אוכלים סלת עגבניות תעים

It sounds like salahtgvinot. Usually I can hear the separate words, is this just me? Is the first time I couldn't hear the separation. And I speak insanely fast. Like my dad still can't understand me & I've been speaking for decades.

עגבניות should read tomatoes since it is plural