Can I overload resistors when they are in parallel?
The power ratings simply add, assuming that each resistor has the same cooling in the set as it would have individually. If they're all bundled close together, you'll have to derate them. So, ten 0.25-watt resistors is equivalent to one 2.5-watt resistor.

If you want to overload them, you might try immersing them in mineral oil. Back when ham radio operators built things like homemade dummy loads, it was said that you could put about 10&times; the power through a resistor if it was immersed in oil. So you could build a very nice, low-inductance 200W dummy load by connecting ten 510-&Omega;, 2-W carbon composition resistors in parallel and immersing the whole assembly in a paint can filled with oil. (IIRC, Heathkit even sold such a kit, called the "Cantenna".)