I'm germinating mandarin, kumquat, citron and orange (Jaffa Shamouti) seeds. I live in Finland so I must have the trees inside all the time. I remember reading that kumquat does not produce good roots and should be grafted to some other tree. Am I correct? How about the other ones? Which of those four is the best rootstock for me?

If somebody has any kind of lime seeds to spare, I'd like to have some. I can't find any in Finland.

Sour orange, trifoliata or swingle are very good rootstock for most types of citrus. You should be able to find Trifoliata in Europe without a problem.

From time to time one hears that Kumquats do not grow well on their own roots. From what I can see, Kumquats do just fine growing on their own root systems. That is not to say that a Kumquat might not do better when grafted onto a special root stock to meet a specific situation such as climate, soil type and other conditions more suitable to a particular type of under stock. This is true for almost all varieties of citrus. Oranges, Mandarins, Lemons, and what have you, will perform better when grown on a particular root stock depending on the environment where the tree is being cultivated. Kumquats have been growing on their own roots for thousands of years, and made it this far. Seedling Kumquats, grown in containers do well growing on their own roots, and produce very sound trees. - Millet

If I grow my oranges etc in a pot inside and thus have to keep them pretty compact size, is there a point in grafting? How much difference would there be with and without grafting? I'm a student and I don't have too much money.

Yes there is. Poncirus Trifoliata (Flying dragon) is a dwarfing rootstock that will keep the tree to a manageable size. You can do this by growing a tree on it's own roots in a small pot. The problem there becomes that if the tree can not reach it's required node count at a small size, it will not fruit.

Kumquats are even smaller than Poncirus trifoliata trees. How about growing an orange on kumquat roots?

I agree with Laaz concerning the use of Flying Dragon as a root stock for container trees. Flying Dragon is a very cold hardy root stock, and a dwarfing root stock. The main difference between growing a grafted tree from a mature bud, and growing a seedling tree from a planted seed is the time required for the tree to produce fruit. A seedling orange could take 10 years before the tree matures and begins to bloom. A grafted orange would begin to bloom in one or two years. The fastest tree to mature and produce eatable fruit when grown from a seed is a Key (Mexican) Lime. Key Lime should fruit in 2-3 year. Mandarins in 3-5 years. - Millet