hiya misdee, yes they can have tomatoes, they can eat most vegetables especially root veg as it helps keep their teeth from getting too long, ours go mad for celery, mango, kiwi, strawberries, tomato etc. \link{http://www.pimms-pages.co.uk/forums/forum_posts.asp?TID=5934&PN=1\this} should help you.
As for going down the ramp, they will learn it very quickly, you need to get them used to your voices so talk to them all the time and give them snacks through the cage, as you've no dandelion leaves you can give them a handful of grass and they'll get used to the fact that your their feeder and it won't be long before they run out and squeak every time they see or hear you.
Make sure that they have an abundance of fresh hay at all times, which food are you giving them?
One of mine was underweight and we started them on \link{http://www.pet-supermarket.co.uk/erol.html#9614x0&&http%3A%252F%252Fwww.google.com%252Fsearch%3Fsourceid%3Dnavclient%26ie%3DUTF-8%26rls%3DGGLD%2CGGLD%3A2005-13%2CGGLD%3Aen%26q%3Dsupa+guinea+excel\this} and they have all changed dramatically, their coats are gorgeous, soft and shiny, you can get the excel from vets or pet shops and all vets recommend it.
If you need any more help just shout.