Does this formula for the amplitude of a Chebyshev filter at cornerfrequentie only hold for even orders?
For Chebyshev responses, the given formulas do apply for n=2 as well as n>2 (even and odd).
Please note that in case n=1 we have a simple first order lowpass response - and it is NOT possible to discriminate between Butterworth, Chebyshev, Bessel,.... 

Background: For Chebyshev filters, the corner frequency is defined at the end of the "ripple band" - however, a first-order response does not exhibit any ripple at all.