This is really two different questions: 1) Yes, you can use Roman Numerals as page numbers. Just click on Insert > Page Number (as you normally would). Press the control key, and click on the page number. Select either lower case or cap. numerals; 2) Pages doesn't look at your page numbers in order to build a ToC. It physically counts the pages, so whether your page numbers are Swahili, Roman or ancient Etruscan numerals, Pages will make them Arabic in the ToC. There are four ToC styles you can choose, and none of them changes the numerals to Roman. Sorry.
No, there isn't. Because ToC text acts like Placeholder text -- click on one word and the entire block of text is selected. So you can't go in and just change the numbers. The only way to get what you want with Pages is to build the ToC manually.